“After an extraordinary amount of reflection and consideration, I am announcing today that I will not be a candidate for reelection to the United States Senate,” Snowe said in a statement. “It has been an indescribable honor and immeasurable privilege to serve the people of Maine.”

Her retirement means the Senate will lose one of its most centrist Republicans — one who often crossed party lines to work with Democrats, and acted as a bridge between the two parties.

It is also a huge boon for Democrats and their fight to maintain control of the upper chamber: they had little hope of beating Snowe, but become heavy favorites to win an open seat in the Democratic-leaning state. Democrats are defending 23 seats to the Republicans’ 10, and this gives them a prime pickup opportunity. The GOP needs a net gain of only four seats (if President Obama wins reelection) to run the Senate.

Snowe, who turned 65 last week, is the 10th senator to retire this cycle and the third Republican. That figure does not include former Sen. John Ensign (R-Nev.), who resigned amid an ethics scandal last year.

Snowe’s decision, coupled with Monday’s news that former Sen. Bob Kerrey (D-Neb.) is reconsidering a run for Senate, has made it a good week for Democrats. If he decides to run for retiring Sen. Ben Nelson’s (D) seat, the race could turn from a Republican slam-dunk into a competitive contest.

The Democratic Senatorial Campaign Committee (DSCC) crowed about Snowe’s retirement Tuesday night.

“As we said from day one, unexpected opportunities will emerge and the DSCC will be in a position to seize on these opportunities,” Executive Director Guy Cecil said in a statement. “Maine is now a top pickup opportunity for Senate Democrats.”

The announcement caught Republicans completely off guard: Snowe didn’t tell Senate Minority Leader Mitch McConnell (R-Ky.) or National Republican Senatorial Committee Chairman John Cornyn (Texas) of her plans until Tuesday afternoon, just hours before she put out her statement, according to a GOP source.

Cornyn said not to count the GOP out of the race.

“While I would never underestimate the fight ahead in defending any open Senate seat, Republicans remain well-positioned to win back a Senate majority in November,” he said in a statement. “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.”

The three-term senator was long expected to run for reelection, and had seemed to be gearing up to do so, with strong fundraising and a top-notch campaign team. 

One former staffer close to Snowe said the senator was worried about her primary, where she faced Tea Party favorite Scott D’Amboise, and that internal poll numbers had indicated she was in real danger.

“She was freaked out about the primary, she was very paranoid that she was going to lose the primary, and what happened in the [recent statewide] caucuses confirmed her fears,” said the former staffer. “She’s definitely a fighter, there’s no question. The poll numbers just were not there.”

D’Amboise turned in strong fundraising numbers, and conservatives had hoped he’d give Snowe a tough run.

In her retirement statement, Snowe ripped Washington’s polarized atmosphere.

“What I have had to consider is how productive an additional term would be,” she said. “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.”

One of the most controversial votes of Snowe’s career was in October of 2009, when she voted with Democrats on the Senate Finance Committee to send Obama’s healthcare bill to the Senate floor.

She was criticized heavily for that move.

But colleagues offered their praise Tuesday evening. Sen. Susan Collins (R-Maine) said she was “absolutely devastated” about Snowe’s retirement.

“I will personally miss her knowledge and her support — but, most of all, I will miss her friendship,” said Collins in a statement. “I wish her my very best.”

Maine’s filing deadline is March 15, which doesn’t give possible replacements much time to make a decision.

There are a few Democrats already in the race — Maine state Rep. John Hinck (D) and former Secretary of State Matthew Dunlap (D) — but the open seat will likely attract more candidates. Possible Democrats include former Rep. Tom Allen (Maine); Rep. Chellie Pingree (Maine), who lost to Collins by 16 points in 2002; and Pingree’s daughter, former state House Speaker Hannah Pingree.

Rep. Pingree thanked Snowe for her service and indicated she’s considering a run.

“This upcoming election is critical to the future of our working families around the country, and in the coming days I will carefully consider how I can best serve the people of Maine,” she said in a statement.

A senior Republican source suggested former Ambassador Peter Cianchette, University of Maine Athletic Director Steve Abbott, Attorney General Bill Schneider and state Treasurer Bruce Poliquin as possible GOP candidates. D’Amboise will also continue his campaign, he announced Tuesday evening.

— This story was last updated at 8:10 p.m. and on March 2 at 2:40 p.m.