Joe Lieberman is the former Democratic Senator from Connecticut. He's both a former Senator and a former Democrat. He was for a long time George W. Bush's go-to guy for a Democrat who would support him on the war; when this cost him the Democratic nomination in 2006, he ran as an independent and won anyway.
The funny thing is that it was his willingness to run as an independent that cost him the nomination.
With the support of Connecticut's Republicans, he barely won re-election to his seat, (by barely we mean by under 10 points). He currently caucuses with the Democrats, supporting Democratic leaders for the Senate. However, he constantly votes with the Republicans on any legislation designed to either end the war in Iraq or to limit the President's ability.
Earlier in his career, he was well known as Al Gore's running mate in 2000 (although the "blame Nader" crowd would prefer you not mention this inconvenient truth), and for being a righteous tightass. He was the only Democrat in the Senate to publicly rebuke Bill Clinton, well except for Robert Byrd, and he privately led a crusade to get professional wrestling off of TV. His most frequently used phrase during 2000 debates with Dick Cheney was "I agree with my opponent on that." In 1988 he first won his seat running against liberal Republican Lowell Weicker by running to the right and seeking and getting the endorsement of, among others, the Moral Majority.
He was very supportive of George W. Bush and his Iraq war. He was recently very supportive of John McCain, and followed him around like a puppy everywhere he went in hopes that he might become vice-president. Didn't happen.
We just wish he'd have gotten it over with and register as a Republican. Unfortunately, other than being a neoconservative warmonger who endorses Republican candidates, "Fighting Joe" Lieberman doesn't have much in the way of GOP credentials. First of all he's not a born again Christian. He voted no on a constitutional ban of same-sex marriage. He co-sponsored the bill giving gay and lesbian federal workers domestic partner benefits. He tried to end job discrimination on the basis of sexual orientation. He gets an "F" from the NRA. He voted against enshrining flag worship in the Constitution. In 2004 he said, "I support affirmative action programs, including in appropriate instances consideration of race and gender in government contracting decisions, when the affirmative action program is designed to remedy the effects of past discrimination." Somehow, however, he not only supported McCain, but also Palin, without reservation. Since he demonstrated his willingness to sell-out every other principle he has ever had in the name of self-interest, it's entirely possible that he would have rebuked all of the above at some point in the future if he was still in the senate. However, he resigned his seat in 2012 to be replaced with an actual Democrat.
There was a great deal of talk about stripping him of all of his seniority as a Democrat (since he still caucuses with them in the Senate) when the most recent Congress started, but apparently Barack Obama (the man he campaigned against, mind you) was more interested in getting Joe to vote with the Democratic Party than in punishing him. It worked for a little bit, he was a reliable Democratic vote except on the Democrats' major issue, health care reform with a public option. On that issue he intends to filibuster. The fact that Connecticut contains the headquarters of most of the insurance industry could possibly be mere coincidence.