Democrats have also expanded the playing field on Republican turf, recruiting former Surgeon General Richard Carmona to run in Arizona and landing Rep. Joe Donnelly, D-Ind., in case Republican Sen. Richard Lugar falls in the primary.
Snowe's decision complicates matters even more. She was well-positioned to cruise to reelection, facing little serious primary opposition, avoiding a top Democratic challenger, and raising over $3 million dollars.
But now Republicans must begin the search for a new candidate. The one Republican in the race -- Scott D'Amboise -- is not viewed as a serious candidate to replace her. Former state Rep. Peter Cianchette, state Senate President Kevin Raye and Collins' former chief of staff Steve Abbott are among the prospective candidates.
Democrats, too, must find a consensus choice, but they are licking their chops tonight in a state Obama won by nearly 20 points in 2008. Rep. Chellie Pingree, D-Maine, announced she would be considering a bid, and Rep. Mike Michaud, D-Maine, released a statement saying he was "humbled by the outpouring of encouragement" from voters urging him to run for the seat. Former Democratic congressman Tom Allen, who badly lost to Sen. Susan Collins in 2008, is another potential candidate.
"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," said Democratic Senatorial Campaign Committee executive director Guy Cecil.
There is not a lot of time for the GOP and Democratic fields to take shape. Snowe announced her decision just over two weeks before the March 15 filing deadline. Senate Republican leadership was caught flat-footed by her announcement, with a GOP source relaying that Snowe did not inform party leaders of her decision until Tuesday afternoon.
Republicans are still in a credible position to win control the Senate, but strategists are privately increasingly nervous about a landscape that once looked riper than it does today. The GOP has recruited well in other key states -- Linda Lingle in Hawaii, Josh Mandel in Ohio, and Heather Wilson in New Mexico -- but the party is far the favorites to control the Senate in 2013.
"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," said National Republican Senatorial Committee Chairman John Cornyn on Tuesday, after Snowe's announcement.