At lunchtime Tuesday, Maine’s U.S. Senate race was looking to be a sleeper. Olympia Snowe, the popular and wealthy Republican moderate who has served in Congress for more than three decades, was expected to trample her Tea Party primary challengers and sail on to an easy victory in the general election.

Then, a few hours later, came the bombshell: Snowe, who had been raising money and campaigning hard, had decided not to seek reelection. The Senate, she said, was no longer the sort of place she wished to spend the next six years of her life.

“With my Spartan ancestry I am a fighter at heart, and I am well prepared for the electoral battle,” Snowe said in a statement released late Tuesday afternoon, adding she was in good health and certain of victory. “However, what I have had to consider is how productive an additional term would be” on account of the “atmosphere of polarization and ‘my way or the highway’ ideologies” that she said had infected the country’s governing institutions.

Snowe’s departure reduces the likelihood that the Republicans will win control of the Senate, where Democrats enjoy a 53–47 majority. Neither of her Tea Party primary challengers ever represented a serious threat, and one of them, Andrew Ian Dodge, withdrew from the GOP in the aftermath of the party’s controversial Feb. 10 presidential caucus, in which party officials initially refused to tabulate results from some counties, and failed to tabulate other results, allegedly due to clerical errors. Any newcomers to the race have just two weeks to collect and certify the necessary signatures to get on the ballot.

“Olympia had to know this is extremely late in the process, and I don’t know what her motives are,” says Jon Reisman, an economist at the University of Maine at Machias and a former Republican congressional nominee. “There are a lot of people in the Republican Party who are unhappy right now.”

Four Democrats were already in the race, three of them present or former state legislators. But more formidable candidates may join them now that the popular incumbent is out of the picture. Maine’s two U.S. representatives—Chellie Pingree and Michael Michaud—are Democrats and both issued statements Tuesday indicating they were considering running for Snowe’s seat. Political observers wonder if they will be joined by Michaud’s current Republican challenger, state Senate president Kevin Raye, which could clear the stage of the Second District congressional race.

“It totally changes the political landscape in Maine,” says former state GOP chair Ted O’Meara. “Anytime something like this happens it’s like a spring that’s been uncoiled, with all the people with political ambitions seeing an opportunity to move up.”

Mr. O’Meara, who has known Snowe for nearly 40 years and managed her 1980 congressional campaign, said she had agonized over the decision. “She’s become increasingly frustrated with the political climate in Washington,” he said. “When she first went to the Senate there were still a lot of moderate Republicans and Democrats who were willing to reach across the aisle on issues where they could find agreement, but it’s become a smaller and smaller group.”

“I suspect she has come to the conclusion that the Senate has become an institution that is incapable of healing itself and that she could have more of an impact on the outside than she could on the inside,” O’Meara added. “I suspect she will be very engaged.”

Snowe’s statement suggested she had no intention of retiring from politics, and included language curiously similar to that of Americans Elect, the Internet-driven presidential nominating organization, and No Labels, which fosters trans-partisan cooperation. “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,” she said. “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.”

Snowe is one of the last representatives of a dying breed, the “Northeast” or “Teddy Roosevelt” Republican: fiscally conservative, environmentally conscious, and liberal or ambivalent on social issues. Once the dominant force in the GOP, this Yankee camp has dwindled as the South has taken control in recent decades. Even in Maine—which also sent GOP moderates Margaret Chase Smith, Bill Cohen, and Susan Collins to the Senate—the party has been tilting rightward, as Tea Party insurgents and social conservatives have gained influence and put the bellicose Paul LePage in the governor’s mansion.

“To some degree, she must have felt hounded in the past couple of years, as she became the face of what the Tea Party sees as unreliable RINOs” or Republicans in name only, says University of Southern Maine political scientist Ron Schmidt. “Does this mean the closure of the era of the Republican Party of Olympia Snowe and the rise of the Republican Party of Paul LePage and the Tea Party? This election strikes me as an opportunity to find out what sort of GOP we have here.”

“Olympia has been trying to be a moderate while the party she has been keeping one foot planted on is an ice floe headed right out to sea,” says state legislator Jon Hinck of Portland, one of the Democratic contenders for Snowe’s seat, who says he personally admires her and her career. “Maybe if she had left the Republican Party when [Vermont Sen.] Jim Jeffords became an independent a few years ago, she’d be in a much better position.”