Maine Republican Sen. Olympia Snowe will not seek reelection in 2012, she announced Tuesday.

View Photo Gallery: A look at Sen. Olympia Snowe’s (R-Maine) career in Washington.

In announcing her plans, Snowe, 65, emphasized that she is in good health and was prepared for the campaign ahead. But she said she was swayed by the increasing polarization in Washington.

“Unfortunately, I do not realistically expect the partisanship of recent years in the Senate to change over the short term,” Snowe said in a statement. “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.”

Snowe’s retirement represents a major setback for the GOP’s efforts to regain a majority in the Senate. As a moderate Republican, she may be the party’s only hope to hold a seat in the strongly blue state.

Republicans did get some traction in the state in 2010, including electing Republican Paul LePage as governor.

But in a more neutral political environment, and in a federal race, Democrats will be heavy favorites to steal this seat from Republicans — their best pickup opportunity in the country, for sure.

Snowe’s announcement took Republican leaders completely by surprise; she informed Senate Minority Leader Mitch McConnell (Ky.) and National Republican Senatorial Committee Chairman John Cornyn (Texas) of her decision today, according to a person familiar with the decision.

The person noted that Snowe had given no indication that she was doing anything but planning to run an aggressive reelection bid; she had hired Justin Brasell, a renowned Republican operative, to manage her campaign.

One Republican said Snowe had sent an invitation for a fundraiser as late as Monday.

Among the names mentioned as possible replacements on the ballot for the GOP: 2002 gubernatorial candidates Peter Cianchette, 2010 gubernatorial candidate Steve Abbott, state Treasurer Bruce Poliquin, state Attorney General Bill Schneider, state Senate President Kevin Raye and Jock McKernan, Snowe’s husband and the state’s former governor. Businessman Scott D’Amboise was already challenging Snowe in a primary.

On the Democratic side, Reps. Chellie Pingree (D) and Mike Michaud (D) could run, as well as former congressman Tom Allen (D), who unsuccessfully challenged Sen. Susan Collins (R-Maine) in 2008.

The filing deadline is fast approaching: March 15.

Snowe is known for her independent streak, often voting with Collins against many of her Republican colleagues. According to the just-released National Journal 2011 vote rankings, only Collins voted with Democrats more than Snowe last year.

The most moderate Democrat in the Senate — Sen. Ben Nelson (D-Neb.) — is also retiring.

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