List of presidents pro tempore of the United States Senate

The president pro tempore of the United States Senate (also president pro tem) is the second-highest-ranking official of the United States Senate. Article I, Section Three of the United States Constitution provides that the vice president of the United States, despite not being a senator, is the president of the Senate. It also establishes that the Senate must choose a president pro tempore to act in the absence of the vice president:^[1]

The Senate shall choose their other Officers, and also a President pro tempore, in the absence of the Vice President, or when he or she shall exercise the Office of President of the United States.

In practice, neither the vice president nor the president pro tempore usually presides; instead, the duty of presiding officer is rotated among junior senators of the majority party to give them experience in parliamentary procedure.

The president pro tempore is third in the line of succession to the presidency, after the vice president and the speaker of the House of Representatives and ahead of the secretary of state.^[2]

Since 1890, the most senior senator in the majority party has generally been chosen to be president pro tempore,^[3] and holds the office continuously until the election of another president pro tempore.^[3] During most of the 62nd Congress, following William Frye's resignation on April 27, 1911, five senators—Augustus Bacon, Charles Curtis, Jacob Gallinger, Henry Cabot Lodge, and Frank Brandegee—alternated as president pro tempore.

Since the office was created in 1789, 92 individuals, from 39 of the 50 states, have served as president pro tempore of the Senate. The current president pro tempore is Patty Murray of Washington, who assumed office on January 3, 2023, at the start of the 118th Congress. In 2001, the honorary title of president pro tempore emeritus was created, and it has been given to a senator of the minority party who has previously served as president pro tempore. This title is currently held by Chuck Grassley of Iowa.

Every president pro tempore but one has been a member of a political party or faction; the number affiliated with each is:

  Democratic – 32;   Republican – 25;   Democratic-Republican – 15;^[a][b]   Federalist – 10;   Jacksonian – 3;^[b]   Anti-Administration – 2;^[a]   National Republican – 2;   Pro-Administration – 2;   Whig – 2;   Independent – 1.

Presidents pro tempore

Portrait	Name	State	Party		Term	Congress
	John Langdon^[c]	New Hampshire		Pro-Administration	April 6–21, 1789	1st Congress
					August 7–9, 1789
	Richard Henry Lee	Virginia		Anti-Administration	April 18 – October 8, 1792	2nd Congress
	John Langdon	New Hampshire		Pro-Administration	November 5 – December 4, 1792
					March 1–3, 1793
					March 4 – December 2, 1793	3rd Congress
	Ralph Izard	South Carolina		Pro-Administration	May 31 – November 9, 1794
	Henry Tazewell	Virginia		Anti-Administration	February 20, 1795 – June 7, 1795
				Democratic-Republican		4th Congress
					December 7–8, 1795
	Samuel Livermore	New Hampshire		Federalist	May 6 – December 4, 1796
	William Bingham	Pennsylvania		Federalist	February 16 – March 3, 1797
	William Bradford	Rhode Island		Federalist	July 6 – October 1797	5th Congress
	Jacob Read	South Carolina		Federalist	November 22 – December 12, 1797
	Theodore Sedgwick	Massachusetts		Federalist	June 27 – December 5, 1798
	John Laurance	New York		Federalist	December 6–27, 1798
	James Ross	Pennsylvania		Federalist	March 1 – December 1, 1799
	Samuel Livermore	New Hampshire		Federalist	December 2–29, 1799	6th Congress
	Uriah Tracy	Connecticut		Federalist	May 14 – November 16, 1800
	John E. Howard	Maryland		Federalist	November 21–27, 1800
	James Hillhouse	Connecticut		Federalist	February 28 – March 3, 1801
	Abraham Baldwin	Georgia		Democratic-Republican	December 7, 1801 – January 14, 1802	7th Congress
					April 17 – December 13, 1802
	Stephen R. Bradley	Vermont		Democratic-Republican	December 14, 1802 – January 18, 1803
					February 25, 1803
					March 2 – October 16, 1803
	John Brown	Kentucky		Democratic-Republican	October 17 – December 6, 1803	8th Congress
					January 23 – February 26, 1804
	Jesse Franklin	North Carolina		Democratic-Republican	March 10 – November 4, 1804
	Joseph Anderson	Tennessee		Democratic-Republican	January 15 – February 3, 1805
					February 28 – March 2, 1805
					March 2 – December 1, 1805
	Samuel Smith	Maryland		Democratic-Republican	December 2–15, 1805	9th Congress
					March 18 – November 30, 1806
					March 2 – October 25, 1807
					April 16 – November 6, 1808	10th Congress
	Stephen R. Bradley	Vermont		Democratic-Republican	December 28, 1808 – January 8, 1809
	John Milledge	Georgia		Democratic-Republican	January 30 – March 3, 1809
					March 4 – May 21, 1809	11th Congress
	Andrew Gregg	Pennsylvania		Democratic-Republican	June 26 – December 18, 1809
	John Gaillard	South Carolina		Democratic-Republican	February 28 – March 2, 1810
					April 17 – December 11, 1810
	John Pope	Kentucky		Democratic-Republican	February 23 – November 3, 1811
	William H. Crawford^[d]	Georgia		Democratic-Republican	March 24, 1812 – March 3, 1813	12th Congress
					March 4–23, 1813	13th Congress
	Joseph B. Varnum	Massachusetts		Democratic-Republican	December 6, 1813 – February 3, 1814
	John Gaillard^[e]	South Carolina		Democratic-Republican	April 18 – November 25, 1814
					November 25, 1814 – December 3, 1815
					December 4, 1815 – March 3, 1817	14th Congress
					March 4, 1817	15th Congress
					March 6, 1817 – February 18, 1818
					March 31, 1818 – January 5, 1819
	James Barbour	Virginia		Democratic-Republican	February 15 – December 5, 1819
					December 6–26, 1819	16th Congress
	John Gaillard	South Carolina		Democratic-Republican	January 25, 1820 – December 2, 1821
					December 3–27, 1821	17th Congress
					February 1 – December 2, 1822
					February 19 – November 30, 1823
					December 1, 1823 – January 20, 1824	18th Congress
					May 21, 1824 – March 3, 1825
					March 9 – December 4, 1825	19th Congress
	Nathaniel Macon	North Carolina		Democratic-Republican	May 20 – December 3, 1826
					January 2 – February 13, 1827
					March 2 – December 2, 1827
	Samuel Smith	Maryland		Jacksonian	May 15 – December 18, 1828	20th Congress
					March 13 – December 10, 1829	21st Congress
					May 20 – December 31, 1830
					March 1 – December 4, 1831
					December 5–11, 1831	22nd Congress
	Littleton Tazewell	Virginia		Jacksonian	July 9–16, 1832
	Hugh Lawson White^[f]	Tennessee		Jacksonian	December 3, 1832 – December 1, 1833
					December 2–15, 1833	23rd Congress
	George Poindexter	Mississippi		National Republican	June 28 – November 30, 1834
	John Tyler	Virginia		National Republican	March 3 – December 6, 1835	24th Congress
	William R. King	Alabama		Democratic	July 1 – December 4, 1836
					January 28 – March 3, 1837
					March 7 – September 3, 1837	25th Congress
					October 13 – December 3, 1837
					July 2 – December 18, 1838
					February 25 – December 1, 1839
					December 2–26, 1839	26th Congress
					July – December 15, 1840
					March 3, 1841
					March 4, 1841	27th Congress
	Samuel Southard^[g]	New Jersey		Whig	March 11, 1841 – May 31, 1842
	Willie P. Mangum^[g]	North Carolina		Whig	May 31, 1842 – December 3, 1843
					December 4, 1843 – March 3, 1845	28th Congress
					March 4, 1845	29th Congress
	Ambrose H. Sevier^[h]	Arkansas		Democratic	December 27, 1845
	David R. Atchison	Missouri		Democratic	August 8 – December 6, 1846
					January 11–13, 1847
					March 3 – December 5, 1847
					February 2–8, 1848	30th Congress
					June 1–14, 1848
					June 26 – July 29, 1848
					July 29 – December 4, 1848
					December 26, 1848 – January 1, 1849
					March 2–4, 1849
					March 5, 1849	31st Congress
					March 16 – December 2, 1849
	William R. King^[i]	Alabama		Democratic	May 6–19, 1850
					July 11, 1850 – March 3, 1851
					March 4, 1851 – December 20, 1852	32nd Congress
	David R. Atchison^[i]^[j]	Missouri		Democratic	December 20, 1852 – March 3, 1853
					March 4, 1853 – December 4, 1854	33rd Congress
	Lewis Cass^[j]	Michigan		Democratic	December 4, 1854
	Jesse D. Bright^[j]	Indiana		Democratic	December 5, 1854 – December 2, 1855
					December 5, 1855 –June 9, 1856	34th Congress
	Charles E. Stuart^[j]	Michigan		Democratic	June 9 – June 10, 1856
	Jesse D. Bright^[j]	Indiana		Democratic	June 11, 1856 – January 6, 1857
	James M. Mason^[j]	Virginia		Democratic	January 6 – March 3, 1857
					March 4, 1857	35th Congress
	Thomas J. Rusk	Texas		Democratic	March 14 – July 29, 1857
	Benjamin Fitzpatrick	Alabama		Democratic	December 7–20, 1857
					March 29 – May 2, 1858
					June 14 – December 5, 1858
					January 19, 1859
					January 25 – February 9, 1859
					March 9 – December 4, 1859	36th Congress
					December 19, 1859 – January 15, 1860
					February 20–26, 1860
	Jesse D. Bright	Indiana		Democratic	June 12 – June 26, 1860
	Benjamin Fitzpatrick	Alabama		Democratic	June 26 – December 2, 1860
	Solomon Foot	Vermont		Republican	February 16–17, 1861
					March 23 – July 3, 1861	37th Congress
					July 18 – December 1, 1861
					January 15, 1862
					March 31 – May 21, 1862
					June 19 – December 12, 1862
					February 18 – March 3, 1863
					March 4 – December 6, 1863	38th Congress
					December 18–20, 1863
					February 23, 1864
					March 11–13, 1864
					April 11–13, 1864
	Daniel Clark	New Hampshire		Republican	April 26, 1864 – January 4, 1865
					February 9–19, 1865
	Lafayette S. Foster^[k]	Connecticut		Republican	March 7, 1865 – March 2, 1867	39th Congress
	Benjamin F. Wade^[k]	Ohio		Republican	March 2–3, 1867
					March 4, 1867 – March 3, 1869	40th Congress
	Henry B. Anthony	Rhode Island		Republican	March 23–28, 1869	41st Congress
					April 9 – December 5, 1869
					May 28 – June 2, 1870
					July 1–5, 1870
					July 14 – December 4, 1870
					March 10–12, 1871	42nd Congress
					April 17 – May 9, 1871
					May 23 – December 3, 1871
					December 21, 1871 – January 7, 1872
					February 23–25, 1872
					June 8 – December 1, 1872
					December 4–8, 1872
					December 13–15, 1872
					December 20, 1872 – January 5, 1873
					January 24, 1873
	Matthew H. Carpenter	Wisconsin		Republican	March 12–13, 1873	43rd Congress
					March 26 – November 30, 1873
					December 11, 1873 – December 6, 1874
					December 23, 1874 – January 4, 1875
	Henry B. Anthony	Rhode Island		Republican	January 25–31, 1875
					February 15–17, 1875
	Thomas W. Ferry^[l]	Michigan		Republican	March 9–10, 1875	44th Congress
					March 19 – December 20, 1875
					December 20, 1875 – March 4, 1877
					March 5, 1877	45th Congress
					February 26 – March 3, 1878
					April 17 – December 1, 1878
					March 3–17, 1879
	Allen G. Thurman	Ohio		Democratic	April 15 – November 30, 1879	46th Congress
					April 7–14, 1880
					May 6 – December 5, 1880
	Thomas F. Bayard, Sr.^[m]	Delaware		Democratic	October 10–13, 1881	47th Congress
	David Davis^[m]	Illinois		Independent	October 13, 1881 – March 3, 1883
	George F. Edmunds^[m]	Vermont		Republican	March 3 – December 2, 1883
					December 3, 1883 – January 14, 1884	48th Congress
					January 14, 1884 – March 3, 1885
	John Sherman^[n]	Ohio		Republican	December 7, 1885 – February 26, 1887	49th Congress
	John James Ingalls^[n]	Kansas		Republican	February 26 – December 4, 1887
					December 5, 1887 – March 3, 1889	50th Congress
					March 7–17, 1889	51st Congress
					April 2 – December 1, 1889
					December 5–10, 1889
					February 28 – March 18, 1890
					April 3, 1890 – March 2, 1891
	Charles F. Manderson	Nebraska		Republican	March 2 – December 6, 1891
					December 7, 1891 – March 3, 1893	52nd Congress
					March 4–22, 1893	53rd Congress
	Isham G. Harris	Tennessee		Democratic	March 22, 1893 – January 7, 1895
	Matt W. Ransom	North Carolina		Democratic	January 7–10, 1895
	Isham G. Harris	Tennessee		Democratic	January 10 – March 3, 1895
	William P. Frye^[o]^[p]	Maine		Republican	February 7, 1896 – March 3, 1897	54th Congress
					March 4, 1897 – December 3, 1899	55th Congress
					December 4, 1899 – March 3, 1901	56th Congress
					March 7, 1901 – March 4, 1903	57th Congress
					March 5, 1903 – March 3, 1905	58th Congress
					March 4, 1905 – March 3, 1907	59th Congress
					December 5, 1907 – March 3, 1909	60th Congress
					March 4, 1909 – April 3, 1911	61st Congress
					April 4–27, 1911	62nd Congress
	Augustus O. Bacon	Georgia		Democratic	August 14, 1911
	Charles Curtis	Kansas		Republican	December 4–12, 1911
	Augustus O. Bacon	Georgia		Democratic	January 15–17, 1912
	Jacob H. Gallinger	New Hampshire		Republican	February 12–14, 1912
	Augustus O. Bacon	Georgia		Democratic	March 11–12, 1912
	Frank B. Brandegee	Connecticut		Republican	March 25–26, 1912
	Augustus O. Bacon	Georgia		Democratic	April 8, 1912
	Jacob H. Gallinger	New Hampshire		Republican	April 26–27, 1912; May 7, 1912
	Augustus O. Bacon	Georgia		Democratic	May 10, 1912
	Henry Cabot Lodge	Massachusetts		Republican	May 25, 1912
	Augustus O. Bacon	Georgia		Democratic	May 30 – June 3, 1912; June 13 – July 5, 1912
	Jacob H. Gallinger	New Hampshire		Republican	July 6–31, 1912
	Augustus O. Bacon	Georgia		Democratic	August 1–10, 1912
	Jacob H. Gallinger	New Hampshire		Republican	August 12–26, 1912
	Augustus O. Bacon^[q]	Georgia		Democratic	August 27 – December 15, 1912
	Jacob H. Gallinger^[q]	New Hampshire		Republican	December 16, 1912, and January 4, 1913
	Augustus O. Bacon^[q]	Georgia		Democratic	January 5–18, 1913
	Jacob H. Gallinger^[q]	New Hampshire		Republican	January 19 – February 1, 1913
	Augustus O. Bacon^[q]	Georgia		Democratic	February 2–15, 1913
	Jacob H. Gallinger^[q]	New Hampshire		Republican	February 16 – March 3, 1913
	James Paul Clarke	Arkansas		Democratic	March 13, 1913 – March 3, 1915	63rd Congress
					December 6, 1915 – October 1, 1916	64th Congress
	Willard Saulsbury Jr.	Delaware		Democratic	December 14, 1916 – March 4, 1917
					March 5, 1917 – March 3, 1919	65th Congress
	Albert B. Cummins^[r]	Iowa		Republican	May 19, 1919 – March 3, 1921	66th Congress
					March 7, 1921 – December 2, 1923	67th Congress
					December 3, 1923 – March 3, 1925	68th Congress
					March 4–6, 1925	69th Congress
	George H. Moses	New Hampshire		Republican	March 6, 1925 – March 4, 1927
					December 15, 1927 – March 3, 1929	70th Congress
					March 4, 1929 – December 6, 1931	71st Congress
					December 7, 1931 – March 3, 1933	72nd Congress
	Key Pittman	Nevada		Democratic	March 9, 1933 – January 2, 1935	73rd Congress
					January 7, 1935 – January 4, 1937	74th Congress
					January 5, 1937 – January 2, 1939	75th Congress
					January 3, 1939 – November 10, 1940	76th Congress
	William H. King	Utah		Democratic	November 19, 1940 – January 3, 1941
	Pat Harrison	Mississippi		Democratic	January 6 – June 22, 1941	77th Congress
	Carter Glass	Virginia		Democratic	July 10, 1941 – January 5, 1943
					January 14, 1943 – January 2, 1945	78th Congress
	Kenneth McKellar^[s]	Tennessee		Democratic	January 6, 1945 – January 2, 1947	79th Congress
	Arthur H. Vandenberg^[s]	Michigan		Republican	January 4, 1947 – January 2, 1949	80th Congress
	Kenneth McKellar^[s]	Tennessee		Democratic	January 3, 1949 – January 2, 1951	81st Congress
					January 3, 1951 – January 2, 1953	82nd Congress
	Styles Bridges	New Hampshire		Republican	January 3, 1953 – January 4, 1955	83rd Congress
	Walter F. George	Georgia		Democratic	January 5, 1955 – January 2, 1957	84th Congress
	Carl Hayden^[t]	Arizona		Democratic	January 3, 1957 – January 6, 1959	85th Congress
					January 7, 1959 – January 2, 1961	86th Congress
					January 3, 1961 – January 8, 1963	87th Congress
					January 9, 1963 – January 3, 1965	88th Congress
					January 4, 1965 – January 9, 1967	89th Congress
					January 10, 1967 – January 2, 1969	90th Congress
	Richard Russell Jr.	Georgia		Democratic	January 3, 1969 – January 3, 1971	91st Congress
					January 3, 1971 – January 21, 1971	92nd Congress
	Allen J. Ellender	Louisiana		Democratic	January 22, 1971 – July 27, 1972
	James Eastland^[u]^[v]	Mississippi		Democratic	July 28, 1972 – January 2, 1973
					January 3, 1973 – January 13, 1975	93rd Congress
					January 14, 1975 – January 3, 1977	94th Congress
					January 4, 1977 – December 27, 1978	95th Congress
	Warren Magnuson	Washington		Democratic	January 15, 1979 – December 4, 1980	96th Congress
	Milton Young	North Dakota		Republican	December 5, 1980
	Warren Magnuson	Washington		Democratic	December 6, 1980 – January 4, 1981
	Strom Thurmond	South Carolina		Republican	January 5, 1981 – January 2, 1983	97th Congress
					January 3, 1983 – January 2, 1985	98th Congress
					January 3, 1985 – January 5, 1987	99th Congress
	John C. Stennis	Mississippi		Democratic	January 6, 1987 – January 3, 1989	100th Congress
	Robert Byrd	West Virginia		Democratic	January 3, 1989 – January 2, 1991	101st Congress
					January 3, 1991 – January 4, 1993	102nd Congress
					January 5, 1993 – January 3, 1995	103rd Congress
	Strom Thurmond	South Carolina		Republican	January 4, 1995 – January 6, 1997	104th Congress
					January 7, 1997 – January 6, 1999	105th Congress
					January 7, 1999 – January 3, 2001	106th Congress
	Robert Byrd	West Virginia		Democratic	January 3–20, 2001	107th Congress
	Strom Thurmond	South Carolina		Republican	January 20 – June 6, 2001
	Robert Byrd	West Virginia		Democratic	June 6, 2001 – January 3, 2003
	Ted Stevens	Alaska		Republican	January 3, 2003 – January 3, 2005	108th Congress
					January 3, 2005 – January 4, 2007	109th Congress
	Robert Byrd	West Virginia		Democratic	January 4, 2007 – January 3, 2009	110th Congress
					January 3, 2009 – June 28, 2010	111th Congress
	Daniel Inouye	Hawaii		Democratic	June 28, 2010 – January 5, 2011
					January 5, 2011 – December 17, 2012	112th Congress
	Patrick Leahy	Vermont		Democratic	December 17, 2012 – January 3, 2013
					January 3, 2013 – January 3, 2015	113th Congress
	Orrin Hatch	Utah		Republican	January 3, 2015 – January 3, 2017	114th Congress
					January 3, 2017 – January 3, 2019	115th Congress
	Chuck Grassley	Iowa		Republican	January 3, 2019 – January 3, 2021	116th Congress
					January 3, 2021 – January 20, 2021	117th Congress
	Patrick Leahy	Vermont		Democratic	January 20, 2021 – January 3, 2023
	Patty Murray	Washington		Democratic	January 3, 2023 – present	118th Congress

Notes

1. Henry Tazewell was affiliated with the Anti-Administration bloc prior to formation of the Democratic-Republican Party.
2. Samuel Smith was a member of the Democratic–Republican Party until it became fractured, at which time he sided with its pro-Jackson faction.
3. Served during a vacancy in the vice presidency, April 6–21, 1789, prior to the inauguration of John Adams.
4. Served during a vacancy in the vice presidency, April 20, 1812–March 4, 1813, following the death of George Clinton.
5. Served during a vacancy in the vice presidency, November 23, 1814–March 4, 1817, following the death of Elbridge Gerry.
6. Served during a vacancy in the vice presidency, December 28, 1832–March 4, 1833, following the resignation of John C. Calhoun.
7. Served during a vacancy in the vice presidency, April 4, 1841–March 4, 1845, after John Tyler succeeded to the presidency.
8. Ambrose H. Sevier was not elected as president pro tempore in an official manner, but permitted to occupy the chair for the day.
9. Served during a vacancy in the vice presidency, July 9, 1850–March 4, 1853, after Millard Fillmore succeeded to the presidency.
10. Served during a vacancy in the vice presidency, April 18, 1853–March 4, 1857, following the death of William R. King.
11. Served during a vacancy in the vice presidency, April 15, 1865–March 4, 1869, after Andrew Johnson succeeded to presidency.
12. Served during a vacancy in the vice presidency, November 22, 1875–March 4, 1877, following the death of Henry Wilson.
13. Served during a vacancy in the vice presidency, September 19, 1881–March 4, 1885, after Chester A. Arthur succeeded to presidency.
14. Served during a vacancy in the vice presidency, November 25, 1885–March 4, 1889, following the death of Thomas A. Hendricks.
15. Served during a vacancy in the vice presidency, November 21, 1899–March 4, 1901, following the death of Garret Hobart.
16. Served during a vacancy in the vice presidency, September 14, 1901–March 4, 1905, after Theodore Roosevelt succeeded to presidency.
17. Served during a vacancy in the vice presidency, October 30, 1912–March 4, 1913, following the death of James S. Sherman.
18. Served during a vacancy in the vice presidency, August 2, 1923–March 4, 1925, after Calvin Coolidge succeeded to presidency.
19. Served during a vacancy in the vice presidency, April 12, 1945–January 20, 1949, after Harry S. Truman succeeded to presidency.
20. Served during a vacancy in the vice presidency, November 22, 1963–January 20, 1965, after Lyndon B. Johnson succeeded to presidency.
21. Served during a vacancy in the vice presidency, October 10, 1973–December 6, 1973, following the resignation of Spiro Agnew.
22. Served during a vacancy in the vice presidency, August 9, 1974–December 19, 1974, after Gerald Ford succeeded to presidency.

President pro tempore emeritus

In 2001, the honorary title of president pro tempore emeritus was created. It has subsequently been bestowed upon a senator of the minority party who has previously served as president pro tempore.^[3]

Portrait	Name	State	Party		Term	Congress
	Strom Thurmond	South Carolina		Republican	June 6, 2001 – January 3, 2003	107th Congress
	Robert Byrd	West Virginia		Democratic	January 3, 2003 – January 3, 2007	108th Congress 109th Congress
	Ted Stevens	Alaska		Republican	January 3, 2007 – January 3, 2009	110th Congress
Vacant January 3, 2009 – January 3, 2015 (No senator was eligible for the position.)						111th Congress 112th Congress 113th Congress
	Patrick Leahy	Vermont		Democratic	January 3, 2015 – January 20, 2021	114th Congress 115th Congress 116th Congress 117th Congress
	Chuck Grassley	Iowa		Republican	January 20, 2021 – present	117th Congress 118th Congress

See also

• Dean of the United States Senate
• History of the United States Senate
• Seniority in the United States Senate
• List of current presidents of legislatures, presiding officers of legislative assemblies worldwide

References

1. Gill, Kathy. "US Senate Organization". The New York Times Company. Retrieved October 19, 2009 – via About.com.
2. Mount, Steve. "Constitutional Topic: Presidential Line of Succession". USConstitution.net. Steve Mount. Retrieved October 19, 2009.
3. Davis, Christopher M. (December 20, 2012). The President Pro Tempore of the Senate: History and Authority of the Office (Report). Congressional Research Service. Retrieved February 23, 2015.

  This article incorporates public domain material from About the President Pro Tempore. United States Senate.