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In a surprise that could reconfigure the fight to control the Senate, Senator Olympia Snowe, a three-term Republican from Maine, said Tuesday she would not run for re-election, citing excessive partisanship in the Senate.

“After 33 years in the Congress this was not an easy decision,” said Ms. Snowe in a prepared statement. “My husband and I are in good health. We have laid an exceptionally strong foundation for the campaign, and I have no doubt I would have won re-election. It has been an indescribable honor and immeasurable privilege to serve the people of Maine, first in both houses of Maine’s legislature and later in both houses of Congress. To this day, I remain deeply passionate about public service, and I cherish the opportunity I have been given for nearly four decades to help improve the lives of my fellow Mainers.”

Ms. Snowe, a moderate who cast key votes in bills that were dear to Democrats including the stimulus bill, was facing a Tea Party-backed challenger, but one who had failed to gain much traction in a state where Ms. Snowe remained popular and well known.

Ms. Snowe said the lack of comity and bipartisanship in the current Congress was a key motivating factor to her sudden retirement, which would well upend Republican efforts to retake the Senate; the party needs four seats to do so.

“I do find it frustrating,” Ms. Snowe said, “that an atmosphere of polarization and ‘my way or the highway’ ideologies has become pervasive in campaigns and in our governing institutions. With my Spartan ancestry I am a fighter at heart; and I am well prepared for the electoral battle, so that is not the issue. However, what I have had to consider is how productive an additional term would be. Unfortunately, I do not realistically expect the partisanship of recent years in the Senate to change over the short term. 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, which is what a fourth term would entail.”

Senator Susan Collins, Republican of Maine and Ms. Snowe’s ally and sometimes rival, said Tuesday afternoon in a prepared statement she was “absolutely devastated” by the news. “Olympia could always be counted on as a leader who sought solutions, not political advantage,” said Ms. Collins, a fellow moderate. “She served our nation with distinction and she continues to bring honor to our state.”