From Wikipedia

Congress

Part Two/Two

The post-Civil War Gilded Age was marked by Republican dominance of the Congress. The Progressive Era saw the Seventeenth Amendment, ratified in 1913, which provided for the direct election of senators. The early 20th century witnessed the rise of strong party leadership in both houses of Congress. In the House of Representatives, the office of speaker became extremely powerful. Leaders in the Senate were somewhat less powerful; individual senators still retained much of their influence. After the revolt against Speaker Joe Cannon in 1910, the seniority system emerged. Members became powerful committee chairmen through years of seniority, regardless of the leadership. Committee chairmen remained particularly strong in both houses until the reforms of the 1970s and 1990s.

The United States Capitol

Franklin D. Roosevelt's election as president in 1932 marked a shift in power towards the presidency. Numerous New Deal initiatives were proposed from the White House and sent to Congress for approval, rather than legislation originating in Congress. After the Watergate scandal and other abuses of power by the Richard Nixon administration, Congress began to reassert its power to oversee the executive branch and develop legislation.

During Roosevelt's administration, from 1933 until 1945, the Democratic Party controlled both houses of Congress. Republicans won control of both houses in the 1946 elections, only to lose them in 1948; with Republican Dwight D. Eisenhower's election to the presidency in 1952, Republicans again won both houses. However, after the Democratic Party again won back control in the elections of 1954, it was the majority party in both houses of Congress for most of the next 40 years; Republicans were only able to win control of the Senate for a six-year period from 1981 until 1987. Republicans won a majority in each house of Congress in the elections of 1994 and controlled both houses until 2006, except for the Senate for most of 2001 and 2002, when the Democrats had the majority after Jim Jeffords left the Republican Party to become an independent and caucus with the Democrats. In 2006, Democrats regained control of the House of Representatives, and the Senate elections yielded a makeup of 49 Democrats, 49 Republicans, and two independents. In the 110th Congress (2007–08), the Democratic voting bloc had a 51-49 majority in the Senate because the two independents, Joseph Lieberman of Connecticut and Bernie Sanders of Vermont, aligned themselves with the Democratic caucus. In the 111th Congress, which convened in 2009, the Democratic Party holds a majority in each house of Congress.