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.