Senate Minority Leader Harry Reid, D-Nev., (left) and Majority Leader Mitch McConnell, R-Ky., at a ceremony last month at the U.S. Capitol. Chip Somodevilla/Getty Images hide caption

itoggle caption Chip Somodevilla/Getty Images

Vice President Biden and House Speaker John Boehner wait for President Obama's State of the Union address in front of a joint session of Congress in 2012. As vice president, Biden is also leader of the U.S. Senate, but only gets a vote when senators are evenly divided. Saul Loeb/AP hide caption

itoggle caption Saul Loeb/AP

Former Massachusetts Sen. Edward William Brooke speaks in the Rotunda on Capitol Hill in Washington, in 2009 during a ceremony where he received the Congressional Gold Medal. Brooke died on Saturday at age 95. Alex Brandon/AP hide caption

itoggle caption Alex Brandon/AP

Republican U.S. Senate candidate Dan Sullivan greets supporters on election night in Anchorage. The as-yet-undecided race between Sullivan and Democratic incumbent Sen. Mark Begich was the hottest in the state. Ted S. Warren/AP hide caption

itoggle caption Ted S. Warren/AP

Colorado Republican Rep. Cory Gardner officially announces his candidacy for the U.S. Senate at Denver Lumber Co. on March 1. Gardner will run against Democratic incumbent Mark Udall. Chris Schneider/AP hide caption

itoggle caption Chris Schneider/AP

Sen. Lindsey Graham of South Carolina voices his opposition to President Obama's choice of former Sen. Chuck Hagel of Nebraska as secretary of defense, on Capitol Hill last week. J. Scott Applewhite/AP hide caption

itoggle caption J. Scott Applewhite/AP