Washington (CNN) -- Republican Sen. Olympia Snowe of Maine announced Tuesday she will retire rather than seek re-election this year, surprising colleagues and perhaps casting the future of her seat in doubt.
The decision was made "after an extraordinary amount of reflection and consideration," she said in a statement.
Snowe, who turned 65 last week, was first elected to the U.S. House in 1978 and then to the Senate in 1994. She is the first woman to serve in both chambers of a state legislature and the U.S. Congress.
Snowe was known as a moderate who sometimes sided with Democrats in the increasingly partisan environment of Washington politics.
Her statement cited the partisan divide.
"I have no doubt I would have won re-election," Snowe said, describing her political service in Maine and Washington as "an indescribable honor and immeasurable privilege."
While her motivation and sense of responsibility remain, she continued, "I do find it frustrating, however, that an atmosphere of polarization and 'my way or the highway' ideologies has become pervasive in campaigns and in our governing institutions."
"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."
The White House released a statement praising Snowe's bipartisan efforts.
"For almost four decades, Olympia Snowe has served the people of the great state of Maine. ... From her unwavering support for our troops, to her efforts to reform Wall Street, to fighting for Maine's small businesses, Sen. 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."
Initial reaction from colleagues indicated Snowe's decision was unexpected.
"I was surprised. I know that she's been quite frustrated with the lack of civility in Washington and the fact that she's a bridge builder -- and it was more and more difficult to do that. But we were all surprised here in Maine that she would make the decision," said Maine GOP Chairman Charlie Webster. "It's going to shake things up in Maine politics."
Maine Sen. Susan Collins, a moderate Republican, said she was "devastated" to learn of Snowe's decision not to seek re-election.
"I know this was an incredibly difficult decision for Olympia," said Collins, who lauded Snowe as "a leader who sought solutions, not political advantage."
Democratic Sen. Chuck Schumer of New York expressed surprise at the news when told by CNN.
"We'll miss her," Schumer said. "She's a good, good lady and an example of ... sometimes how the roughness of the political world can affect things. She's great, and she'll be missed by people on both sides of the aisle."
With Snowe in the race, two of the top political handicappers -- the Cook Political Report and the Rothenberg Political Report -- had considered the seat "safe Republican" in the November election.
Democrats hold a 53-47 majority in the Senate but are defending 23 of the 33 seats up for grabs in November, including those of two independents who caucus with the Democrats.
Guy Cecil, executive director of the Democratic Senatorial Campaign Committee, called Snowe's departure an "unexpected opportunity."
"Maine is now a top pick-up opportunity for Senate Democrats," Cecil said in a statement. "If there is one place in the country that is likely to reject the extreme, anti-middle class, divisive Republican agenda it is Maine. Democrats not only hold a strong registration advantage in the state, but this is a state that the president won by 17 points in 2008 and will likely win by a significant margin this year, as well."
However, Texas Sen. John Cornyn, the chairman of the National Republican Senatorial Committee, said his party remains "well-positioned to win back a Senate majority in November."
"Maine has a proud history of electing independent leaders, including a Republican governor in 2010, and while this will be a key battleground in the fall, I am confident it will remain in Republican hands," Cornyn said in a statement.
The Maine GOP chairman said it's too early to tell yet whether Democrats could potentially pick up the seat.
"We've got a number of qualified people that could run," said Webster.
Two Republican candidates already announced primary challenges to Snowe -- former Lisbon Falls Selectman Scott D'Amboise and tea party activist Andrew Ian Dodge.
In addition, two Democrats being mentioned as potential candidates were businesswoman and 2010 gubernatorial candidate Rosa Scarcelli and state Rep. Emily Cain.
U.S. Rep. Chellie Pingree, D-Maine, was also hailed as a possible candidate. The Progressive Change Campaign Committee launched a campaign shortly after Snowe's announcement to draft the Democratic representative into running. The group, which claims 6,000 members in Maine and close to a million nationwide, praised Pingree for having a "bold progressive voice."
CNN's Ted Barrett, Deirdre Walsh, Rebecca Stewart, Tom Cohen and Shannon Travis contributed to this report.