STAMFORD–In announcing his retirement Wednesday, U.S. Sen. Joseph I. Lieberman acknowledged the challenges he would have faced if he sought re-election in 2012 without the backing of a major party, but denied that the stark realities of electoral politics dissuaded him from one last campaign.

“I know that some people have said that if I ran for re-election, it would be a difficult campaign for me. But what else is new?” Lieberman said, smiling and holding his hands out in a half-shrug. “It probably would be.”

But it is a fight Lieberman will skip, retiring after four terms. After losing a Democratic primary and then winning as an independent in 2006, he will not explore the difficult terrain of a three-way race, one in which he could expect strong Democratic and Republican opponents.

Lieberman, who turns 70 in February 2012, arrived with three generations of Liebermans at 12:40 p.m. in a crowded conference room in a Marriott that he says was built in an urban renewal project near the spot where he lived with his family in a cold-water flat until he was eight years old.

He said the location made him think of his own personal journey, from a modest upbringing in Stamford to Yale, the Connecticut General Assembly, Congress and nomination as the first Jewish candidate for vice president, and that of his four immigrant grandparents.

“They came to America hoping for opportunity, and they got it,” he said. “But even they could not have dreamed that their grandson would end up as a U.S. senator and a barrier-breaking candidate for vice president.”

In his 22-minute speech, the Democrat-turned-independent paid homage to John F. Kennedy, an iconic leader of the Democratic Party he left in 2006, for stirring an early interest in public service. He looked back over a 40-year career in Connecticut politics and ahead to his last two years in office and whatever comes next.

“I do not intend today to be the end of my career in public service. Having made this decision not to run enables me to spend the next two years in the Senate devoting the full measure of my energy and attention to getting things done for Connecticut and for our country,” Lieberman said, reading from a text at a lectern, surrounded by his wife, Hadassah, children and grandchildren.

Text of Lieberman’s speech

He was composed, jovial at times. He joked that he once promised Hadassah he would stay in the Senate until Regis Philbin gave up his daytime television talk show. Philbin announced Wednesday that he would retire at age 79. His standing-room audience laughed.

An Orthodox Jew who routinely missed Democratic state nominating conventions that conflicted with the Sabbath, Lieberman relied on scripture to explain his reason for retiring.

“The reason I have decided not to run for re-election in 2012 is best expressed in the wise words of Ecclesiastes: ‘To everything there is a season, and a time for every purpose under Heaven.’ At the end of this term, I will have served 24 years in the U.S. Senate and 40 years in elective office. For me, it is time for another season and another purpose under Heaven.”

He offered no other reason for making his announcement now, a decision that will leave Connecticut’s congressional delegation with a lame duck senator and a freshman, Richard Blumenthal.

Lieberman is set to exit the political stage as both a divisive and unifying figure in American politics, a man the left came to detest for his support of the war in Iraq, but was admired, at times grudgingly, for a pivotal role he played in the Senate as an independent.

“Even if we don’t always see eye to eye, I always know Joe is coming from a place of principle,” President Obama said in a statement. “I know he will carry with him that integrity and dedication to his remaining work in the Senate and to whatever he chooses to do next.”

On health care reform, Lieberman had both undercut and supported Obama, delivering a fatal blow to the public option sought by the president and many other Democrats, then helping salvage the amended bill with his vote. He promised to go his own way over his last two years in the Senate. In 2006, Obama campaigned for Lieberman in Connecticut. In the 2008 presidential race, Lieberman rejected Obama for John McCain, a Republican.

“I will keep doing everything in my power to build strong bridges across party lines — to keep our country safe, to win the wars we are in, and to make sure America’s leadership on the world stage is principled and strong. I will keep doing everything I can to keep our economy growing and get our national debt under control, to combat climate change, to end our dependency on foreign oil, and to reform our immigration laws,” he said.

“And when my Senate chapter draws to a close in 2013, I look forward to new opportunities that will allow me to continue to serve our country-and to stay engaged and involved in the causes that I have spent my career working on, and that I care so much about.”

He smiled to the applause of a crowd that reflected his career, faces from campaigns distant and recent. He waved and exited the room, not taking questions from reporters.

Lieberman glossed over this estrangement from the Democratic Party that elected him to the state Senate, the attorney general’s office, the U.S. Senate and almost to the office of vice president. His loss in a Democratic primary in 2006 was a painful blow, and he never reconciled with the Connecticut Democratic Party, even as he remained a member of the Democratic caucus in the U.S. Senate.

Roy Occhiogrosso, his media adviser during the failed 2006 primary, was in the audience, as was Dan Gerstein, who succeeded him after Lieberman continued as a petitioning candidate. Jim Kennedy, who worked on Lieberman’s campaign for attorney general and joined him in Washington, was off to the side. He played a role in crafting the speech.

“Along the way, I have not always fit comfortably into conventional political boxes–Democrat or Republican, liberal or conservative,” Lieberman said. “I have always thought that my first responsibility is not to serve a political party but to serve my constituents, my state, and my country, and then to work across party lines to make sure good things get done for them. Whatever the partisan or policy differences that divide us, they are much less important than the shared values and dreams that unite us and that require us to work together to make progress for all. To me, that is what public service and leadership is all about.”

Also in the audience was Daniel Papermaster, who as a young former aide to Christopher Dodd, literally showed Lieberman how to find his way to the Senate floor for the first time in 1989. Papermaster later served as legal counsel to Lieberman’s campaigns.

There was Waterbury Mayor Michael Jarjura, a Democrat who stood with Lieberman on the steps of city hall in Waterbury in 2006, where Lieberman kicked off his campaign as an independent in 2006 after losing the primary to the antiwar candidate, Ned Lamont.

Andrew J. McDonald, the former state senator from Stamford, said his relationship with the Lieberman families goes back to the days when Lieberman’s mother, Marcia, helped McDonald’s mother win a seat on the board of education in Stamford.

McDonald, who is gay, was invited by Lieberman to witness President Obama sign the repeal of “Don’t Ask, Don’t Tell,” the policy that forced gays to either lie about their sexual orientation or face expulsion. Lieberman led the fight for repeal.

Gerstein called the speech “a classic Lieberman statement,” forward-looking, optimist and full of love of country.

“I know for some people, it comes off as corny,” Gerstein said. “But it’s Lieberman.”

Occhiogrosso, now a senior adviser to Gov. Dannel P. Malloy, the former mayor of Stamford, said he came from Hartford out of affection that survived Lieberman’s break with many in the Democratic Party.

“I like him. I don’t always agree with him. I don’t know anyone who does,” Occhiogrosso said. “But he’s a decent person and he’s always been gracious to me. That should mean something.”