66th Congress (1919-1921)
Majority Leader: None.
Minority Leader: Oscar W. Underwood (D-AL)

Note: Oscar W. Underwood became the first elected party leader on April 27, 1920. There was no elected Republican floor leader prior to 1925. During the 66th Congress, Henry Cabot Lodge (R-MA) was the party conference chairman and served as an unofficial party leader.

67th Congress (1921-1923)
Majority Leader: None.
Minority Leader: Oscar W. Underwood (D-AL)

Note: Henry Cabot Lodge (R-MA) continued to serve as unofficial Republican leader.

68th Congress (1923-1925)
Majority Leader: Charles Curtis (R-KS)
Minority Leader: Joseph T. Robinson (D-AR)

Note: Henry Cabot Lodge (R-MA) died on November 9, 1924. Charles Curtis was elected Republican floor leader on March 5, 1925. The Democratic party elected Joseph T. Robinson as floor leader on December 3, 1923.

69th Congress (1925-1927)
Majority Leader: Charles Curtis (R-KS)
Minority Leader: Joseph T. Robinson (D-AR)

70th Congress (1927-1929)
Majority Leader: Charles Curtis (R-KS)
Minority Leader: Joseph T. Robinson (D-AR)

Note: Charles Curtis resigned his Senate seat on March 3, 1929, having been elected Vice President of the United States.

71st Congress (1929-1931)
Majority Leader: James E. Watson (R-IN)
Minority Leader: Joseph T. Robinson (D-AR)

Note: James E. Watson was elected Republican leader on March 5, 1929.

72nd Congress (1931-1933)
Majority Leader: James E. Watson (R-IN)
Minority Leader: Joseph T. Robinson (D-AR)

Note: James Watson lost his reelection bid in 1932 and left office on March 3, 1933.

73rd Congress (1933-1935)
Majority Leader: Joseph T. Robinson (D-AR)
Minority Leader: Charles L. McNary (R-OR)

Note: Charles L. McNary was elected Republican leader on March 7, 1933.

74th Congress (1935-1937)
Majority Leader: Joseph T. Robinson (D-AR)
Minority Leader: Charles L. McNary (R-OR)

75th Congress (1937-1939)
Majority Leader: Joseph T. Robinson (D-AR); Alben Barkley (D-KY)
Minority Leader: Charles L. McNary (R-OR)

Note: Joseph T. Robinson died on July 14, 1937. Alben Barkley was elected Democratic leader on July 22, 1937.

76th Congress (1939-1941)
Majority Leader: Alben Barkley (D-KY)
Minority Leader: Charles L. McNary (R-OR)

Note: In 1940, at the request of Senator McNary, Senator Warren R. Austin (R-VT) served as acting leader.

77th Congress (1941-1943)
Majority Leader: Alben Barkley (D-KY)
Minority Leader: Charles L. McNary (R-OR)

78th Congress (1943-1945)
Majority Leader: Alben Barkley (D-KY)
Minority Leader: Wallace H. White, Jr. (R-ME)

Note: Although Charles McNary continued to be officially listed as minority leader until his death on February 25, 1944, Wallace H. White, Jr. served as acting leader during McNary's illness and was elected Republican leader on January 4, 1945. He retired from the Senate on January 3, 1949.

79th Congress (1945-1947)
Majority Leader: Alben Barkley (D-AR)
Minority Leader: Wallace H. White, Jr. (R-ME)

80th Congress (1947-1949)
Majority Leader: Wallace H. White, Jr. (R-ME)
Minority Leader: Alben Barkley (D-KY)

Note: Alben Barkley resigned his Senate seat on January 19, 1949, having been elected Vice President of the United States.

81st Congress (1949-1951)
Majority Leader: Scott W. Lucas (D-IL)
Minority Leader: Kenneth S. Wherry (R-NE)

Note: Scott W. Lucas was elected Democratic leader on December 31, 1948, to be in effect on January 20, 1949. Lucas lost his bid for reelection in 1950 and left office on January 3, 1951. Kenneth W. Wherry was elected Republican leader on January 3, 1949.

82nd Congress (1951-1953)
Majority Leader: Ernest W. McFarland (D-AZ)
Minority Leader: Kenneth S. Wherry (R-NE); Styles Bridges (R-NH)

Note: Ernest W. McFarland was elected Democratic leader on January 2, 1951. Kenneth Wherry died on November 29, 1951. Styles Bridges was elected Republican leader on January 8, 1952. He chose not to continue as party leader in the 83rd Congress, instead becoming president pro tempore and chair of the Committee on Appropriations.

83rd Congress (1953-1955)
Majority Leader: Robert A. Taft (R-OH); William F. Knowland (R-CA)
Minority Leader: Lyndon B. Johnson (D-TX)

Note: Robert Taft was elected Republican leader on January 2, 1953. He died the following July 31. William Knowland was elected Republican leader on August 4, 1953. Lyndon Johnson was elected Democratic leader on January 2, 1953. William Knowland was the youngest majority leader in Senate history, being elected to the position at the age of 45. When Lyndon Johnson became majority leader two years later, he was 46 years old, ranking him second behind Knowland. Johnson was the youngest Democratic floor leader.

84th Congress (1955-1957)
Majority Leader: Lyndon B. Johnson (D-TX)
Minority Leader: William F. Knowland (R-CA)

85th Congress (1957-1959)
Majority Leader: Lyndon B. Johnson (D-TX)
Minority Leader: William F. Knowland (R-CA)

Note: William Knowland retired from the Senate at the end of the 85th Congress.

86th Congress (1959-1961)
Majority Leader: Lyndon B. Johnson (D-TX)
Minority Leader: Everett M. Dirksen (R-IL)

Note: Lyndon Johnson resigned his Senate seat on January 3, 1961, having been elected Vice President of the United States. Everett Dirksen was elected Republican leader on January 7, 1959.

87th Congress (1961-1963)
Majority Leader: Mike Mansfield (D-MT)
Minority Leader: Everett M. Dirksen (R-IL)

Note: Mike Mansfield was elected Democratic leader on January 3, 1961, and served until January 3, 1977, making him the longest-serving majority leader in Senate history.

88th Congress (1963-1965)
Majority Leader: Mike Mansfield (D-MT)
Minority Leader: Everett M. Dirksen (R-IL)

89th Congress (1965-1967)
Majority Leader: Mike Mansfield (D-MT)
Minority Leader: Everett M. Dirksen (R-IL)

90th Congress (1967-1969)
Majority Leader: Mike Mansfield (D-MT)
Minority Leader: Everett M. Dirksen (R-IL)

91st Congress (1969-1971)
Majority Leader: Mike Mansfield (D-MT)
Minority Leader: Everett M. Dirksen (R-IL); Hugh D. Scott, Jr. (R-PA)

Note: Everett Dirksen died on September 7, 1969. Hugh Scott was elected Republican leader on September 24, 1969.

92nd Congress (1971-1973)
Majority Leader: Mike Mansfield (D-MT)
Minority Leader: Hugh D. Scott, Jr. (R-PA)

93rd Congress (1973-1975)
Majority Leader: Mike Mansfield (D-MT)
Minority Leader: Hugh D. Scott, Jr. (R-PA)

94th Congress (1975-1977)
Majority Leader: Mike Mansfield (D-MT)
Minority Leader: Hugh D. Scott, Jr. (R-PA)

Note: Mike Mansfield and Hugh Scott both retired from the Senate at the end of the 94th Congress.

95th Congress (1977-1979)
Majority Leader: Robert C. Byrd (D-WV)
Minority Leader: Howard H. Baker, Jr. (R-TN)

Note: Robert C. Byrd was elected Democratic leader on January 4, 1977. Howard H. Baker, Jr., was elected Republican leader on January 4, 1977.

96th Congress (1979-1981)
Majority Leader: Robert C. Byrd (D-WV)
Minority Leader: Howard H. Baker, Jr. (R-TN)

97th Congress (1981-1983)
Majority Leader: Howard H. Baker, Jr. (R-TN)
Minority Leader: Robert C. Byrd (D-WV)

98th Congress (1983-1985)
Majority Leader: Howard H. Baker, Jr. (R-TN)
Minority Leader: Robert C. Byrd (D-WV)

Note: Howard Baker retired from the Senate at the end of the 98th Congress.

99th Congress (1985-1987)
Majority Leader: Robert Dole (R-KS)
Minority Leader: Robert C. Byrd (D-WV)

Note: Robert Dole was elected Republican leader on November 28, 1984, effective January 3, 1985.

100th Congress (1987-1989)
Majority Leader: Robert C. Byrd (D-WV)
Minority Leader: Robert Dole (R-KS)

Note: Robert Byrd resigned as majority leader to become chair of the Senate Committee on Appropriations in the 101st Congress and was elected president pro tempore on January 3, 1989.

101st Congress (1989-1991)
Majority Leader: George J. Mitchell (D-ME)
Minority Leader: Robert Dole (R-KS)

Note: George Mitchell was elected Democratic leader on November 29, 1988, effective January 3, 1989.

102nd Congress (1991-1993)
Majority Leader: George J. Mitchell (D-ME)
Minority Leader: Robert Dole (R-KS)

103rd Congress (1993-1995)
Majority Leader: George J. Mitchell (D-ME)
Minority Leader: Robert Dole (R-KS)

Note: George Mitchell retired from the Senate at the end of the 103rd Congress.

104th Congress (1995-1997)
Majority Leader: Robert Dole (R-KS); Trent Lott (R-MS)
Minority Leader: Thomas A. Daschle (D-SD)

Note: On December 22, 1995, Senator Robert Dole broke Charles McNary's record as longest-serving Republican leader, having served since January 3, 1985, ten years, eleven months, and nine days. Dole resigned from the Senate on June 11, 1996, to devote time to his presidential campaign. Trent Lott was elected Republican leader on June 12, 1996. Thomas Daschle was elected Democratic leader on December 2, 1994.

105th Congress (1997-1999)
Majority Leader: Trent Lott (R-MS)
Minority Leader: Thomas A. Daschle (D-SD)

106th Congress (1999-2001)
Majority Leader: Trent Lott (R-MS)
Minority Leader: Thomas A. Daschle (D-SD)

107th Congress (2001-2003)
Majority Leader: Thomas A. Daschle (D-SD)
Minority Leader: Trent Lott (R-MS)

Note: From January 3 to January 20, 2001, with the Senate divided evenly between the two parties, the Democrats held the majority due to the deciding vote of outgoing Democratic Vice President Al Gore. Senator Thomas A. Daschle served as majority leader at that time. Beginning on January 20, 2001, Republican Vice President Richard Cheney held the deciding vote, giving the majority to the Republicans. Senator Trent Lott resumed his position as majority leader on that date. On May 24, 2001, Senator James Jeffords of Vermont announced his switch from Republican to Independent status, effective June 6, 2001. He announced that he would caucus with the Democrats, giving that party a one-seat advantage and changing control of the Senate back to the Democrats. Thomas A. Daschle again became majority leader on June 6, 2001. Trent Lott resigned as Republican leader on December 20, 2002. Bill Frist was elected on Dec. 23, 2002, as Republican Leader for the 108th Congress

108th Congress (2003-2005)
Majority Leader: William H. Frist (R-TN)
Minority Leader: Thomas A. Daschle (D-SD)

109th Congress (2005-2007)
Majority Leader: William H. Frist (R-TN)
Minority Leader: Harry Reid (D-NV)

110th Congress (2007-2009)
Majority Leader: Harry Reid (D-Nevada)
Minority Leader: Mitch McConnell (R-Kentucky)

Sources:

U.S. Congress. Senate. Majority and Minority Leaders of the Senate, by Floyd M. Riddick, S. Doc 100-29, 100th Congress, 2d session, 1988.

Byrd, Robert C. The Senate, 1789-1989: Addresses on the History of the United States Senate. Volume II (Washington: U.S. GPO, 1991).

Baker, Richard A. and Roger H. Davidson, eds. First Among Equals: Outstanding Senate Leaders of the Twentieth Century (Washington, DC: Congressional Quarterly, 1991).

U.S. Congress. Senate. Minutes of the Senate Republican Conference: Sixty-second Congress through Eighty-eighth Congress, 1911-1964, edited by Wendy Wolff and Donald A. Ritchie. Washington: GPO, 1999, Senate Document 105-19.

U.S. Congress. Senate. Minutes of the Senate Democratic Conference: Fifty-eighth through Eighty-eighth Congresses, 1903-1964, edited by Donald A. Ritchie. Washington, GPO, 1999. Senate Document 105-20.

U.S. Congress. Senate Web site: http://www.senate.gov/artandhistory/history/common/briefing/Majority_Minority_Leaders.htm#2