Updated: 6:48 p.m. ET
Republican Senator Olympia Snowe, of Maine, will not seek re-election, she announced on Tuesday.
"After an extraordinary amount of reflection and consideration, I am announcing today that I will not be a candidate for re-election to the United States Senate," she said in prepared statement.
Snowe, a three-term senator and former eight-term House member, said she was "well prepared" for another electoral bid, attributing her decision instead to frustrations with the partisan nature of Congress.
"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," the statement said.
"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."
She said she found the "atmosphere of polarization and 'my way or the highway' ideologies" currently pervading Congress "frustrating."
The longtime lawmaker is widely considered one of the more moderate voices in the Republican party, and has shown willingness to defy party leadership and vote with Democrats on a number of key issues. She supports abortion rights, and was one of few Republicans to help craft President Obama's health care overhaul. Ultimately, however, she did not vote for the president's health care reform bill.
In the 111th Congress, from January 3, 2009 to January 3, 2011, Snowe voted with Republicans only 58.2 percent of the time, according to the Washington Post.
In her statement, Snowe said she and her husband are in "good health" and that she had "no doubt" she 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," she said. "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."
In a statement following Snowe's announcement, Democratic Senatorial Campaign Committee (DSCC) Executive Director Guy Cecil said the organization would be "in position" to "seize on" the opportunity Snowe's retirement will present to Senate Democrats, who are attempting to hold on to a slim 53-47 majority in the Senate.
"As we said from day one, unexpected opportunities will emerge and the DSCC will be in a position to seize on these opportunities," Cecil said in the statement. "Maine is now a top pick up opportunity for Senate Democrats. If there is one place in the country that is likely to reject the extreme, anti-middle class, divisive agenda Republican agenda it is Maine."
According to the New York Times' Nate Silver, Snowe's retirement will dramatically increase Democrats' chances to pick up a seat in coming election cycle. Last December, Silver calculated that Republicans had an 85 percent chance of holding onto their Senate seat in Maine in 2012, based on Snowe's popularity; In a Tweet on Tuesday, he estimated that the figure would drop to "20-30" percent in light of her decision not to seek re-election.
Democrats are defending 23 Senate seats in the 2012 election, including six open seats; Republicans are defending only ten.
According to the Portland Press-Herald, Tea Party-affiliated candidate Andrew Ian Dodge dropped a Senate primary bid against Snowe just last week, saying he would instead attempt to get on the ballot as an Independent.
Scott D'Amboise, who is also affiliated with the Tea Party, is now the only remaining Republican in the race.
According to the Maine Republican party, candidates have until March 15 to collect the 2,000 signatures needed to get on the Republican ballot.
Snowe's decision marks the second piece of potential good news for Senate Democrats this week: According to the Associated Press, Bob Kerrey, a former Senator and Governor in Nebraska, is considering running for the state's open Senate seat. Kerrey, a Democrat, is widely seen as the party's best chance to retain that seat upon the departure of moderate Democrat Ben Nelson, who that he would not seek re-election.