Sen. Olympia J. Snowe, the iconic Republican moderate from Maine, announced her retirement from the Senate on Tuesday, saying she would not seek a fourth term because political partisanship has made the Senate unproductive.

“Unfortunately, I do not realistically expect the partisanship of recent years in the Senate to change over the short term,” Snowe said. “So at this stage of my tenure in public service, I have concluded that I am not prepared to commit myself to an additional six years in the Senate.”

Snowe has made a reputation, over 33 years in Congress, as someone eager to build political bridges between moderates from both parties. But in recent years, she has become an increasingly isolated voice in a Congress hobbled by partisan gridlock.

Snowe’s biography is the stuff of a Capra movie. By the time she was 10, both her parents had died — her mother of breast cancer and her father of a heart attack. She was taken in by her aunt and uncle, and shortly after graduating from the University of Maine, she married Peter Snowe, a state legislator. He died in a car crash in 1973.

At 26, she ran for and won his seat, and she was elected to Congress from the state’s northern House district in 1978. Once on the Hill, she began dating the other member of Maine’s House delegation, John “Jock” McKernan; they became the state’s leading power couple when he went on to become governor, and they married in 1989.

Given her continued popularity, her retirement is a rebuke to the partisanship that has come to define the political times in Washington. Snowe’s willingness to compromise and work with the other side has frequently put her out of step with her party, and Democrats have often looked to her as one of the Republicans willing to break ranks with the GOP leadership. Snowe’s reputation as a moderate has grown as fewer and fewer senators on either side of the aisle have been willing to take bipartisan actions.

But it has also become harder to get elected by advocating moderation and compromise. In recent years, Republican senators in particular have been under pressure from tea party elements in their party to abandon the middle or face primary challenges from the right.

In a sign of the increasingly unique place Snowe holds in the Senate, President Obama issued a statement Tuesday evening praising his former Senate colleague. “Senator Snowe’s career demonstrates how much can be accomplished when leaders from both parties come together to do the right thing for the American people,” he said.

Snowe, 65, comes from a long tradition of moderate New England Republicans, dating to the 1950s, when her political idol, the late Maine senator Margaret Chase Smith, led the effort to rebuke McCarthyism. Though they had some success in the 2010 midterm elections, the moderate GOP’s ranks in New England could be reduced to just two senators and possibly no House members after November.

Snowe is the sixth moderate senator — two Republicans and four Democrats — to announce that they will not seek reelection this year.

“The loss of all these moderates is a real blow to bipartisanship,” former senator John Breaux (D-La.) said Tuesday.

When Snowe arrived in the Senate in 1995, she was part of a bipartisan group, led by Breaux and the late John H. Chafee (R-R.I.), that regularly crafted its own budget and health-care proposals. The group didn’t often win, but some of its ideas were merged into other proposals to entice swing votes.

“That’s where the balance of power was those days, and that’s not true anymore,” Breaux said.

In 2005, Snowe joined the “Gang of 14,” a mostly moderate group made up equally of Democrats and Republicans who defied their party leaders and forced a compromise on filibuster rules in the Senate. By January, only five of those 14 senators will still be in office.

In early 2009, when Democrats held 58 seats in the Senate and needed just a few GOP votes, Snowe teamed up with Republican Sens. Susan Collins (Maine) and Arlen Specter (Pa.) to draw down the size of Obama’s stimulus proposal to less than $800 billion. They were excoriated for their votes by conservatives, and Specter eventually switched parties, while conservatives sought a tea party challenge for Snowe’s 2012 race.

In announcing her decision to leave, Snowe emphasized that she is in good health and was prepared for the campaign. GOP strategists said she was in no danger of losing the Republican primary, because no strong challenger had emerged and Democrats had privately conceded that they did not intend to challenge her.

In a presidential election year — Obama won Maine by more than 15 percentage points in 2008 — Democrats are favored to steal Snowe’s seat from Republicans, their best pickup opportunity in the country.

Snowe’s announcement took Republican leaders by surprise; she informed Senate Minority Leader Mitch McConnell (Ky.) and National Republican Senatorial Committee Chairman John Cor­nyn (Tex.) of her decision on Tuesday, according to a person familiar with it. Snowe’s campaign manager, Justin Brasell, is a close McConnell ally who ran his 2008 reelection bid.

A large part of Snowe’s middle-of-the-road reputation and her frustration with partisanship was built on her role in the marathon negotiations over Obama’s health-care proposal in 2009. A senior member of the Finance Committee, Snowe was one of three Republicans who joined three Democrats in the offices of the chairman, Sen. Max Baucus (D-Mont.).

She was the only Republican on the committee to support the plan.

“Is this bill all that I would want?” Snowe said before the vote. “Far from it. Is it all that it can be? No. But when history calls, history calls. And I happen to think that the consequences of inaction dictate the urgency of Congress to take every opportunity to demonstrate its capacity to solve the monumental issues of our time.”

By December 2009, however, she opposed the bill — as did every other Republican — complaining that Majority Leader Harry M. Reid (D-Nev.) was rushing the massive legislation before an artificial Christmas deadline. Snowe was furious when, a couple of weeks later, Reid told the New York Times that it was “a waste of time dealing with her.”

“I see a vital need for the political center in order for our democracy to flourish and to find solutions that unite rather than divide us,” Snowe said Tuesday. “It is time for change in the way we govern, and I believe there are unique opportunities to build support for that change from outside the United States Senate.”