WASHINGTON — He was the first Jewish American named to a major-party presidential ticket, and he came within a single Supreme Court vote in 2000 of becoming the vice president. Years later, he was seriously considered for the same spot on the ticket – of the other party.

Joe Lieberman will retire Thursday, ending his 24-year term in the U.S. Senate and in a political world that’s changed around him. He exits as a voice often without an echo, an independent without a comfortable spot in either political party, a man in the middle of a political system that prizes partisanship over moderation.

Lieberman usually voted with Democrats, yet many Democrats can’t erase the memory of his enthusiastic support for the Bush administration’s Iraq War effort. They remember how the onetime head of the Connecticut presidential campaign for liberal icon Robert Kennedy in 1968 stumped for Republican conservative John McCain 40 years later, how he wound up addressing the 2008 Republican National Convention and criticizing Democratic nominee Barack Obama.

Lieberman, 70, has had constants: unwavering devotion to a muscular defense and stronger environmental and civil rights laws, and a thoughtful approach to issues. The Connecticut senator also will be known, former aide Dan Gerstein said, for his conscience and character.

Faith and fierce ambition have long motivated the deeply religious Lieberman, ambition that’s sparked charges that his positions are too malleable, his style too calculating.

Lieberman sat in his Senate office recently, explaining his drive.

“I have a lot of values put in me by my upbringing, which was a religious upbringing, and the whole idea we are blessed to be here,” he said. “With that comes a responsibility to do tikkun olam, to make the world better. I chose, for various reasons, public service as a way to try to do that. Of course, you can’t do it unless you’re in office.”

He insisted that ambition did not trump faith when convenient. “If you calculate every decision you make based on how it’s going to affect your re-election, you’re not going to end up doing very much,” he said.

Lieberman was taken aback when offered an example of what critics call his calculating style. He gained national attention in 1998 for his Senate speech branding President Bill Clinton’s behavior in the Monica Lewinsky sex scandal as "immoral" and "disgraceful,” but he didn’t urge impeachment. Critics called that vintage Lieberman, getting national attention but stopping short of a bolder step.

“I thought . . . the president’s action didn’t deserve impeachment,” Lieberman recalled. Impeachment is a way to eventually oust someone from office, he explains, but Clinton remained popular.

That’s trademark Lieberman logic, and sometimes it was a problem. He thought trouble could be overcome by an appeal to reason and a confidence that the world worked in logical ways. Opponents often saw him as a master of calculation, not commitment.

"He was pretty good on labor and social issues, but he never met a war he didn’t like. The Democratic Party will not miss him," said Robert Borosage, a co-founder of the liberal Campaign for America’s Future.

Lieberman likes to say that life is a series of chapters, and his Washington story had four. His rise began with his arrival in the Senate in 1989 and continued through the 2000 election. He was a different Democrat, aligning himself with Clinton as a center-left alternative to liberal party orthodoxy.

Though he lost the vice presidency in 2000, he returned to the Senate and never hinted at even a trace of dismay.

“ ‘You just lost an election. You didn’t lose your life, and you’ve got a lot of years ahead of you when you can do a lot of things,’ ” he recalled his mother saying.

“I’m not haunted by it,” he said. “Do I go back to it periodically and feel disappointed, frustrated, angry? Of course I do."

Work and faith were his salves, and this next chapter was arguably the most productive of his Senate career. As the chairman of the Senate committee that oversees homeland security, Lieberman led the fight, and worked closely with Republicans, to revamp the nation’s domestic anti-terrorist efforts and create the independent commission that investigated the Sept. 11, 2001, attacks.

The Bush administration resisted the commission. “We went up against the status quo and challenged the status quo,” Lieberman recalled, grinning.

Those triumphs were quickly clouded. His bid for the 2004 Democratic presidential nomination went nowhere, and he became the Democrats’ most visible supporter of the increasingly unpopular Iraq War. By 2006, even his home-state support had eroded, and he had a primary challenge for his seat from businessman Ned Lamont.

“I thought, ‘I’m a Democrat. I’ve been a Democrat all my life,’ ” Lieberman said. “I understand how strongly they want us to get out of Iraq quickly, but, you know, I worked so closely with so many Democrats, so I’m going to take this (Senate primary) on. . . . I’m not going to leave on my own.”

They did want to kick him out. He lost the primary on what he called “the most hurtful night of my career.”

That night also proved to be the beginning of a new, bumpier, chapter.

He ran for the Senate as an independent. “Election night . . . maybe I even felt better and more excited that night than I did the first night I got elected to the Senate,” he recalled, as he won easily.

Lieberman returned to the Senate a pariah of sorts. He caucused with Democrats. But by the time the 2008 presidential campaign rolled around, he felt the draw of his friendship with Sen. McCain, R-Ariz., and supported him, a stunning break for a man who’d been on the Democratic ticket just eight years earlier.

McCain reportedly came close to picking Lieberman as the Republican vice-presidential candidate before settling on Sarah Palin as more acceptable to his party’s conservative base.

.Lieberman praised McCain and Palin – and ripped into Obama – in his convention speech. “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,” he said of then-Sen. Obama.

“As I look back at it,” Lieberman said recently, “I wish I’d left those lines out. It wasn’t my purpose there.”

A lot of Democrats saw the speech as an act not of bipartisanship, but of treachery. Adding to Lieberman’s new world was a new kind of Senate, in which he wasn’t roundly embraced by either party and key legislation passed because Democrats and their strong majority stuck together, rarely reaching across aisles.

"There’s no middle anymore," Senate Historian Donald Ritchie said. "That’s why people like Lieberman are now seen as mavericks."

Lieberman leaves on a somber note. He delivered his farewell address Dec. 12, and while full of lofty recollections, he conceded that his quest for common ground remained unfulfilled.

"Today, I regret to say, as I leave the Senate," he said, "the greatest obstacle standing between us and the brighter American future we all want is right here in Washington."

He’ll be succeeded by Democratic Rep. Christopher Murphy of Connecticut, who in the most recent Congress voted with Democrats consistently.