Maine Sen. Olympia Snowe’s surprise retirement announcement Tuesday blindsided fellow Republicans and gave Democrats another morale boost when it comes to their prospects for holding the majority this fall.

Her decision to forgo what would have been an easy re-election campaign is also the latest blow to moderates in the chamber — specifically Northeastern Republicans — whose ranks have been dwindling.

GOP sources said Snowe’s decision caught party leaders completely flat-footed. She did not inform Senate Minority Leader Mitch McConnell (R-Ky.) and National Republican Senatorial
Committee Chairman John Cornyn (Texas) of the news until Tuesday afternoon.

“I am absolutely devastated to learn that Olympia has decided not to seek re-election,” Maine Sen. Susan Collins, a fellow GOP moderate, said in a statement. 

Senate Republican aides were also stunned by the retirement, citing her love of the institution and prime position to take the head of the powerful Commerce Committee if the GOP regained the majority next Congress.

But sources suggested there had been hints of Snowe’s disenchantment with the direction of the Republican Party that tell the story of the moderate’s abrupt departure.

“I think she just doesn’t fit this place anymore,” one Republican aide said. “She’s, in a lot of ways, a very introverted person, and she doesn’t have a lot of people around who she used to be close with. Who is she there with, who does she have connections with?”

The aide continued: “She’s not exactly been a Senator who’s fit in or tried really hard to fit in with her colleagues in the Conference, which is very different from Sen. Collins.”

The ranks of Republican moderates in the Senate have been hit hard in recent cycles by retirements and primary losses. If Massachusetts Sen. Scott Brown (R) loses his re-election bid to Democrat Elizabeth Warren, the Snowe retirement means Collins and Sen. Kelly Ayotte (R-N.H.) could be the only Senate Republicans left from the Northeast next year.

Sen. Bernie Sanders (I-Vt.), a fellow New Englander, said Tuesday he was “surprised” and saddened to hear of Snowe’s announcement.

“I knew Olympia Snowe when we served together in the House, and I think she is one of the most respected Members of the Senate and surely is one of the very last of the Republican moderates,” Sanders said just off the Senate floor.

Snowe was extraordinarily valuable to Democrats during the 111th Congress, when she supported multiple extensions of unemployment benefits, Wall Street reform and worked extensively in the Finance Committee to write what ultimately would become the Democrats’ signature health care law.

Snowe had hired Justin Brasell, a top Republican operative who ran McConnell’s last campaign, to manage her race, and she had $3.4 million in the bank. By all accounts, sources said, Snowe was all systems go.

Her decision leaves Republicans scrambling, with only two weeks to go before the March 15 filing deadline. One GOP consultant familiar with Maine described the situation as “mass chaos.”

“On the Republican side, they don’t really have anyone in waiting, so this is a thunderclap,” Maine Republican strategist Christian Potholm said. “But I think the Republican establishment and Republican forces will certainly not leave that race uncontested.”

Republican names being floated immediately after Snowe’s announcement included former gubernatorial candidate and Ambassador Peter Cianchette; ex-state House GOP Leader Josh Tardy; University of Maine Athletic Director Steve Abbott; state Treasurer Bruce Poliquin; and state Attorney General Bill Schneider. Scott D’Amboise was a long-shot candidate already running for the GOP nomination.

Cornyn still sounded optimistic that the party could net the four seats needed to take control of the chamber.

“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,” Cornyn said in a statement.

The most prominent declared Democratic candidate is former Maine Secretary of State Matt Dunlap. He officially announced his candidacy in December.

But with Snowe out, all eyes are on Reps. Chellie Pingree and Mike Michaud to run for the Democratic nomination. Pingree, a two-term Congresswoman, unsuccessfully challenged Collins in 2002.

“In the coming days, I will carefully consider how I can best serve the people of Maine,” Pingree said in a statement, adding that she called Snowe on Tuesday.

Michaud said in a statement that he is seriously considering running as well.

Other Democratic names mentioned include Pingree’s daughter, former state Speaker Hannah Pingree and former Rep. Tom Allen, who lost to Collins in 2008.

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

Independent candidates are also seriously in the mix, including former gubernatorial candidate Eliot Cutler and former Gov. Angus King. King remains popular in Maine, and Cutler nearly beat Gov. Paul LePage (R) in 2010. Both would have campaign apparatuses at the ready, should they decide to pull the trigger on a Senate bid.

In her statement, Snowe said her decision was not about electoral fears, but about Washington partisanship.

In 2010, several moderate Republicans lost primaries to tea-party-backed challengers. After the midterms, Snowe topped tea party lists of potential targets. But LePage quickly endorsed her and essentially inoculated her from a challenge from the right. The two share a long personal history.

LePage was described as being “saddened” by the news.

Snowe was a political superstar in her home state. She never won a Senate race with less than 60 percent of the vote, and she won her last race in 2006 with 74 percent of the vote.

Kyle Trygstad, Joshua Miller and Shira Toeplitz contributed to this report.