OK, I admit I had to look it up via Answer.com. But it was chewing at me all day.
I was one cycle off. Forbes, a congressman from New York, switched in 1999, not 2001 as I indicated last night. He was defeated in the primary during his next re-election bid, by 35 votes against a 71-year-old woman.
Here is a list of notable party switchers in United States political history, via Answer.com:
http://www.answers.com/topic/party-switchingDemocrat to Republican
1964 - J. Strom Thurmond, while U.S. senator from South Carolina
1983 - W. Philip Gramm, while U.S. representative from Texas (resigned before switching parties and re-won his seat in special election as a Republican)
1994 - Richard C. Shelby, while U.S. senator from Alabama
1995 - Jimmy Hayes, while U.S. representative from Louisiana
1995 - Greg Laughlin, while U.S. representative from Texas
1995 - Ben Nighthorse Campbell, while U.S. senator from Colorado
1995 - Wilbert J. "Billy" Tauzin, while U.S. representative from Louisiana
2001 - Michael Bloomberg, before running for mayor of New York City
2004 - Ralph Hall, while U.S. representative from Texas
2004 - Rodney Alexander, while U.S. representative from Louisiana (his switch just before the filing deadline prevented the Democratic party from fielding a viable candidate in the race)
Though he never formally changed his affiliation, outgoing U.S. Senator Zell B. Miller (D-Georgia), caucuses with the Senate Republicans and spoke at the 2004 Republican National Convention. Former Democratic New York City mayor Ed Koch also announced his support for Bush.
Democrat to third party/independent
1970 - Harry F. Byrd, Jr. conservative Democratic US senator from Virginia became an independent and was reelected to the Senate against opponents from both parties.
2000 - Matt Gonzalez, to the Green Party, during his campaign for the San Francisco Board of Supervisors
2003 - Matt Ahearn, to the Green Party, while a New Jersey state legislator
Republican to Democrat
1971 - John Lindsay, while mayor of New York City
1971 - Leon Panetta, switched parties while not in or running for public office. He later became a U.S. representative from California (1976) and White House Chief of Staff (1994).
1973 - Don Riegle, while U.S. representative from Michigan
1999 - Michael Forbes, while U.S. representative from New York
Republican to third party/independent
1912 Theodore Roosevelt former President of the United States, left the Republican Party after a failed attempt to be nominated for President again. He ran as the candidate of the newly formed Progressive Party, better known as the Bull Moose Party, and received second place, doing better than the Republican candidate but being defeated by the Democratic candidate.
1937 or 1938 Vito Marcantonio a former liberal Republican congressman from New York left the party after being defeated for reelection, and joined the American Labor Party. He was then reelected to Congress.
1952 - Wayne Morse, while U.S. senator from Oregon. He then switched from independent to Democrat in 1956.
1972 - Roger MacBride, went from Republican to Libertarian and back to Republican
1988 - Ron Paul, a former Republican congressman, ran for President as a Libertarian. He later returned to Congress as a Republican.
1990 - Walter Hickel, before his successful bid for Governor of Alaska, switched to the Alaskan Independence Party. He rejoined the Republican party in 1994.
1990 - Lowell P. Weicker, before running for governor of Connecticut
2001 - James M. Jeffords, while U.S. senator from Vermont. This move changed the balance of power in the Senate from 50-50, with Republican Vice President Richard B. Cheney casting the tie-breaking vote and thus providing a "51" majority, to 50-49-1, giving the Democrats majority control of the Senate until it was lost after the 2002 midterm elections.