While we try to catch our breath over Arlen Specter's stunning switch to the Democratic Party, here's a list of the other Senate party switchers in the past 50 years:
Joe Lieberman of Connecticut -- Democrat to independent (2006): Lieberman, the Democratic nominee for vice president in 2000, had been on the outs with many in his party over the war in Iraq, which he supported. In the August 2006 primary, Lieberman was defeated by Ned Lamont, an anti-war candidate. Lieberman then ran as an independent, won re-election, and remained as an independent. And while he continues to vote with the Democrats to organize the Senate, he angered many in the party when he endorsed GOP presidential candidate John McCain in 2008 and appeared at the Republican convention that year. (See "Lamont Pulls Off Upset; What's Next for Lieberman," Political Junkie, Aug. 9, 2006.)
James Jeffords of Vermont -- Republican to independent (May 24, 2001): Perhaps the most significant party switch in Senate history. At the time, the Senate party division was 50 Republicans and 50 Democrats, with the GOP controlling the chamber by way of Vice President Cheney's tie-breaking vote. When Jeffords switched, he gave the Dems a 51-49 voting majority -- an advantage they held until the 2002 elections. Jeffords retired in 2006. (See "The Seismic Result of Jeffords' Decision," Political Junkie, May 25, 2001.
Bob Smith of New Hampshire -- Republican to independent (July 13, 1999): Smith was seeking the Republican nomination for president, a fact that few people were aware of. So he decided to leave the GOP and pursue his White House aspirations as a third-party candidate. (That didn't go so well either.) When Sen. John Chafee (R-RI), the chairman of the Senate Environment Committee, died, Smith switched back to the GOP and, as the most senior Republican on the committee, was given the chairmanship. But voters back home weren't as forgiving, and Smith was defeated in the 2002 GOP primary by Rep. John Sununu.
Ben Nighthorse Campbell of Colorado -- Democrat to Republican (March 3, 1995): His switch came months after the Republicans won control of the Senate in the 1994 elections. He was re-elected as a Republican in 1998 and retired in 2004.
Richard Shelby of Alabama -- Democrat to Republican (Nov. 9, 1994): His switch came one day after the Republicans won control of the Senate in the 1994 elections. He was re-elected as a Republican in 1998 and 2004.
Harry Byrd Jr. of Virginia -- Democrat to independent (1970): Byrd, son of the longtime Virginia Democratic powerhouse Harry Byrd Sr., objected to the state party's implementation of a loyalty oath and became an independent. He was re-elected as an independent in 1970 and 1976 and retired in 1982.
Strom Thurmond of South Carolina -- Democrat to Republican (Sept. 16, 1964): Thurmond, a conservative, found himself more attuned to the views of Barry Goldwater, the GOP presidential candidate that year, whom he endorsed, than those of President Lyndon Johnson. Thurmond was re-elected as a Republican in 1966, 1972, 1978, 1984, 1990 and 1996, retiring in 2002.
Here's the rest of the list of party switchers since direct election of senators was instituted in 1913:
Wayne Morse of Oregon -- Republican to independent (1952) to Democrat (1955)
Robert La Follette Jr. of Wisconsin -- Republican to Progressive (1934) to Republican (1946)
Henrik Shipstead of Minnesota -- Farmer-Labor to Republican (1940)
George Norris of Nebraska -- Republican to independent (1936)
Miles Poindexter of Washington -- Republican to Progressive (1912) to Republican (1915)