Lieberman's independent streak and especially his needling of Democratic presidential nominee Barack Obama during the 2008 presidential contest rankled many Democrats, the party he aligned with in the Senate. Yet his support for gay rights, civil rights, abortion rights and environmental causes at times won him the praise of many liberals over the years.

“In an era of political carbon copies, Joe Lieberman was a singularity. One of one," said Connecticut Sen. Chris Murphy, a Democrat. "He fought and won for what he believed was right and for the state he adored.”

Over the last decade, Lieberman helped lead No Labels, a centrist third-party movement that has said it will offer as-yet-unnamed candidates for president and vice president this year. Some groups aligned with Democrats oppose the effort, fearing it will help presumptive Republican nominee Donald Trump win the White House.

Lieberman came tantalizingly close to winning the vice presidency in the contentious 2000 presidential contest that was decided by a 537-vote margin victory for George W. Bush in Florida after a drawn-out recount, legal challenges and a Supreme Court decision. He was the first Jewish candidate on a major party’s presidential ticket and would have been the first Jewish vice president.

He was also the first national Democrat to publicly criticize President Bill Clinton for his extramarital affair with a White House intern.

Lieberman sought the Democratic presidential nomination in 2004 but dropped out after a weak showing in the early primaries. Four years later, he was an independent who was nearly chosen to be McCain’s running mate. He and McCain were close pals who shared hawkish views on military and national security matters.

McCain was leaning strongly toward choosing Lieberman for the ticket as the 2008 GOP convention neared, but he chose Sarah Palin at the last minute after “ferocious” blowback from conservatives over Lieberman’s liberal record, according to Steve Schmidt, who managed McCain’s campaign.

Lieberman generated controversy in 1998 when he scolded Clinton, his friend of many years, for “disgraceful behavior” in an explosive speech on the Senate floor during the height of the scandal over his relationship with Monica Lewinsky. Yet Lieberman later voted against the impeachment of Clinton.

He defended his partisan switches as a matter of conscience, saying he always had the best interests of Connecticut voters at heart. Critics accused him of pursuing narrow self-interest and political expediency.

In announcing his retirement from the Senate in 2013, Lieberman acknowledged that he did “not always fit comfortably into conventional political boxes” and felt his first responsibility was to serve his constituents, state and country, not his political party. He had a tortured relationship with Democrats.

During his final Senate speech, Lieberman urged Congress to look beyond party lines and partisan rancor to break Washington gridlock.

“It requires reaching across the aisle and finding partners from the opposite party,” said Lieberman. “That is what is desperately needed in Washington now.”

Harry Reid, who served as Senate Democratic leader, once said that while he didn’t always agree with the independent-minded Lieberman, he respected him.

“Regardless of our differences, I have never doubted Joe Lieberman’s principles or his patriotism,” Reid said. “And I respect his independent streak, as it stems from strong convictions.”

Privately, some Democrats were often less charitable about Lieberman’s forays across party lines, which they saw as disloyal. He bolted his party and turned independent after a 2006 Senate primary loss in Connecticut.

Lieberman’s strong support of the Iraq War hurt his statewide popularity. Democrats rejected Lieberman and handed the 2006 primary to a political newcomer and an anti-war candidate, Ned Lamont, who is now serving his second term as Connecticut governor. Lieberman went on to win reelection as an independent.

In a statement issued Wednesday expressing condolences, Lamont said he and Lieberman eventually became friends after the grueling race.

“While the senator and I had our political differences, he was a man of integrity and conviction, so our debate about the Iraq War was serious,” Lamont said in a statement. “I believe we agreed to disagree from a position of principle."

“When the race was over, we stayed in touch as friends in the best traditions of American democracy. He will be missed," he added.

Defying Democratic leaders and friends, Lieberman ran successfully for reelection as an independent and drew support from some Republican allies. Lieberman won praise from the White House and fundraising help from prominent Republicans, such as then-New York Mayor Mike Bloomberg, who himself later ran as an independent.

Lieberman made his Senate experience and congressional clout a strong selling point, saying he’d fight hard for the state’s defense jobs and its fair share of federal largesse. The strategy paid off.

Lieberman won reelection to a fourth term, even though many of his Democratic allies and longtime friends, including former Sen. Chris Dodd, supported Lamont. Lieberman was candid about what he considered a betrayal by old pals such as Dodd, but the two men later reconciled.

Longtime friend and former chairman of the Connecticut Democratic Party, Nick Balletto said many rank-and-file Democrats were unhappy with Lieberman but credited the former senator for dedicating his life to public service and the state.

“He was the most genuine, honest, straightforward, politician you’d probably ever meet. What you saw is what you got,” said Balletto. “His issues were the issues of the people."

After his rebound reelection in 2006, Lieberman decided to caucus with Democrats in the Senate, who let him head a committee in return because they needed his vote to help keep control of the closely divided chamber. But it wasn’t long until Lieberman was showing his independent streak and ruffling his Democratic caucus colleagues.

Despite the decision of Democrats to let him join their caucus as an independent, Lieberman was an enthusiastic backer of McCain in the 2008 presidential contest.

Lieberman’s speech at the 2008 GOP presidential nominating convention criticizing Obama, the Democratic presidential nominee, struck a deep nerve with many Democrats.

Lieberman cast Obama as a political show horse, a lightweight with a thin record of accomplishment in the Senate despite his soaring eloquence as a speaker.

“In the Senate, during the 3 1/2 years that Sen. Obama has been a member, he has not reached across party lines to ... accomplish anything significant, nor has he been willing to take on powerful interest groups in the Democratic Party to get something done,” Lieberman said at the convention.

“Eloquence is no substitute for a record,” he said.

Lieberman campaigned heartily across the country for McCain. Many Democrats considered it a betrayal to Obama and his former party colleagues.

“Joe Lieberman has said things that are totally irresponsible when it comes to Barack Obama,” House Democratic leader Nancy Pelosi of California said in a radio interview during the 2008 race.

After the election, there was speculation Senate Democrats might strip Lieberman of his chairmanship of the Senate Homeland Security and Governmental Affairs Committee as payback. Sen. Patrick Leahy, D-Vt., chair of the Judiciary Committee, was among those who said Lieberman should lose his chairmanship. Leahy branded Lieberman’s attacks on Obama as “beyond the pale.”

But at Obama’s urging, Senate Democrats decided not to punish Lieberman for supporting McCain and the GOP ticket. Obama was eager to strike a bipartisan tone for his presidency and giving Lieberman a pass helped reinforce that message.

Yet Vermont Sen. Bernie Sanders, an independent and staunch liberal, called it a “slap in the face” for millions of Americans who backed Obama.

Lieberman was known in the Senate for his hawkish foreign policy views, his pro-defense bent and his strong support for environmental causes.

Five weeks after the Sept. 11, 2001, attacks, he became one of the first politicians to call for the ouster of Iraqi leader Saddam Hussein and later voted in favor of the military invasion of Iraq. His vocal support for the war would later help doom his candidacy in the 2006 Connecticut Democratic primary.

Lieberman tended to vote with Democrats on most issues and was a longtime supporter of abortion rights, a stance that would have proved problematic with conservatives had McCain chosen him as his running mate in 2008.

He played a key role in the legislation that created the Department of Homeland Security.

Lieberman grew up in Stamford, Connecticut, where his father ran a liquor store. Lieberman graduated from Yale University and Yale Law School in New Haven. As Connecticut’s attorney general from 1983 to 1988, he was a strong consumer and environmental advocate. Lieberman vaulted into the Senate by defeating moderate Republican incumbent Lowell Weicker in 1988.

After leaving the Senate in 2013, Lieberman joined a New York City law firm. His funeral will be held Friday at Congregation Agudath Sholom in his hometown of Stamford. An additional memorial service will be announced at a later date.

Lieberman and his wife, Hadassah, have four children.

Former Associated Press writer Andrew Miga contributed to this report.