He's been an small-i independent since 2006.
Interesting stuff at http://en.wikipedia.org/wiki/Joe_Lieberman
Many Democrats wanted Lieberman to be stripped of his chairmanship of the Senate Committee on Homeland Security and Governmental Affairs due to his support for John McCain which went against the party's wishes.[50] Republican Minority Leader Mitch McConnell reached out to Lieberman, asking him to caucus with the Republicans.[51] Ultimately, the Senate Democratic Caucus voted 42 to 13 to allow Lieberman to keep chairmanship (although he did lose his membership for the Environment and Public Works Committee). Subsequently, Lieberman announced that he will continue to caucus with the Democrats.[6] Lieberman credited President-elect Barack Obama for helping him keep his chairmanship. Obama had privately urged Democratic Senate Majority Leader Harry Reid not to remove Lieberman from his position. Reid stated that Lieberman's criticism of Obama during the election angered him, but that "if you look at the problems we face as a nation, is this a time we walk out of here saying, 'Boy did we get even'?" Senator Tom Carper of Delaware also credited the Democrats' decision on Lieberman to Obama's support, stating that "If Barack can move on, so can we."[52][53]
Partisan members of the Democratic caucus were reportedly angry at the decision to not punish Lieberman more severely. Senator Bernie Sanders of Vermont (who is also an Independent who caucuses with the Democrats) stated that he voted to punish Lieberman "because while millions of people worked hard for Obama, Lieberman actively worked for four more years of President Bush's policies."[53]
Lieberman's embrace of certain conservative policies and in particular his endorsement of John McCain have been cited as factors for his high approval rating among Republicans in Connecticut with 66% of Republicans approving of him along with 52% of independents also approving of his job performance, this however is also cited for his low approval rating among Democrats: 44% approving and 46% disapproving.[54] Currentlly 51% of voters approve of his performance along with 40% disapproving.[54]