Lieberman's popularity once seemed unassailable. A popular state senator and attorney general for over a decade before serving in the U.S. Senate, he won his first three elections to federal office by wide margins. Former Vice President Al Gore chose Lieberman as his running mate in the 2000 election, making Lieberman the first Jewish candidate to be tapped for the vice-presidential ticket. Lieberman was viewed as a political asset and helped Gore compete in Florida, but it wasn't enough. In 2004, Lieberman tried and failed to win the Democratic nomination for president.
Lieberman's hawkish positioning on Iraq alienated his liberal base and, in 2006, motivated anti-war activist Ned Lamont to challenge him in the Democratic primary. Lamont went on to win, but Lieberman ran as an independent and prevailed in the general election. He would later call himself an "Independent Democrat."
After the 2006 race, Lieberman continued to champion the war in Iraq and helped push through legislation creating the Department of Homeland Security. His party affiliation suddenly tenuous, Lieberman then made a decision that would rupture his relationship with the Democratic Party to an irreparable degree: he endorsed Sen. John McCain (R-Ariz.) during the presidential campaign, and campaigned on his behalf.
Lieberman continues to caucus with the Democrats, and clings to leadership positions on several prominent committees including the Homeland Security and Governmental Affairs Committee. But he no longer attends Democratic caucus leadership strategy meetings, and has been the target of considerable criticism from party leaders.
Lieberman's most recent attempts to distinguish himself as a legislator have met with mixed success. A joint effort alongside Sen. John Kerry (D-Mass.) and Lindsey Graham (R-S.C.) to push landmark climate change legislation through Congress failed to gain traction. Yet Lieberman took a leadership role in the successful repeal of the military's 'Don't Ask, Don't Tell' policy during the 111th Congress' lame-duck session, working alongside Sen. Susan Collins (R-Maine).
Sophie Quinton contributed to this report.