The usually mild-mannered Connecticut senator has launched some of the 2004 campaign's harshest attacks on rivals from his own party, accusing them of moving left to embrace "the failed solutions of the past".
Lieberman says Democrats face a choice between candidates such as Howard Dean, whose stark liberal message and naivety in foreign affairs makes him unelectable, and his own more pragmatic, conservative approach that gives him broader, election-winning appeal.
But many Democrats see Lieberman's readiness to compromise and conservative, hawkish tendencies as one reason why their party has struggled to deliver a clear alternative to the Republicans.
Joseph Isadore Lieberman was born on 24 February 1942 in Stamford, Connecticut, the state where he was raised and educated, and which he represents in the Senate today.
A young liberal activist, Lieberman graduated from Yale University in 1964 and Yale Law School in 1967. After practising law for three years, Lieberman stood in the Connecticut state Senate election in 1970 and won.
Age: 61 (24 February 1942)Religion: Jewish (Orthodox)
Current post: Senator Connecticut (1988-present)
Military record: None. Student during Vietnam
After 10 years of state politics, Lieberman tried for a seat in the House of Representatives in 1980, but failed amid a nationwide Republican surge.
After a run as Connecticut's attorney general from 1982 to 1988, Lieberman made it to Congress at the second attempt when he entered the national Senate.
Despite his generally genial manner, Lieberman's strong moral instincts led him to attack fellow Democrat Bill Clinton during the Monica Lewinsky sex scandal.
But he later joined forces with Clinton's deputy in 2000 when Al Gore picked him to be his vice presidential nominee.
Lieberman remains a high profile senator. Besides being the top Democrat on the Senate Governmental Affairs Committee, he sits on the Environment and Public Works, Armed Services and Small Business committees.
Lieberman takes a relatively liberal view on issues such as abortion and the environment. But the deeply religious senator is widely seen as the most conservative of the Democrat presidential contenders, especially given his hawkish support for increasing domestic security activities and missile defence.
He voted for the Patriot Act to increase domestic security but has criticised its implementation, saying many Arab-Americans and those who looked like them had been unjustly detained and mistreated.
Lieberman also wants to use technological improvements to boost business and make the US self-sufficient in oil within 20 years.
Actively pro-war, Lieberman was one of the two Senate Democrats who co-sponsored the 1998 Iraqi Liberation Act, committing the US to changing the Iraqi regime. But he criticises Bush for failing to take a more internationalist approach to rebuilding the occupied country.
Lieberman is an unswerving supporter of Israel, favours increasing aid to the Jewish state, moving the US embassy to Jerusalem and removing Yasir Arafat from his position as leader of the Palestinians.
Lieberman backs Bush's war but
seeks wider help to rebuild Iraq
When Democrat rival Howard Dean – a pro-Israeli politician himself – suggested the US should be more even handed as a mediator, Lieberman called him "irresponsible" for suggesting any compromise on American support for Israel.
Lieberman urges a more internationalist approach to foreign policy than the Bush administration. He wants the US to help "spread democracy" in the Middle East, build stronger relations with China, but tighten trade sanctions on Cuba.
"A Democratic president may have to defy both America's allies and his domestic political base to aggressively fight terrorism and defend freedom. So far, at least, Dean's record on the national stage suggests he doesn't understand that. Lieberman does."
- The New Republic magazine
"The Lieberman persona is so inventive, has been so creative, has been so gymnastic in its many shapes and forms, that only he can even begin to explain it."
- Editorial, The Hartford Courant
Many Democrats, especially on on the left-liberal wing, say Lieberman's record of pragmatic compromise and aggressive stance on defence issues means he is not sufficiently committed to the party's liberal values. Others say the senator lacks passion and charisma.