Olympia Snowe’s announcement last week not to seek a fourth term to represent Maine in the U.S. Senate surprised many in her home state and in Washington. Democrats are now hoping to win the Maine seat, just last week considered an easy win for Republicans.

Maine has voted for Democrats for president every four years since 1992, voting for Barack Obama over John McCain by 18 points in 2008. With Obama at the top of the ticket this fall, the thinking goes, candidates down-ballot will be helped. Democrats outnumber Republicans among registered voters by nearly 50,000, too. Not for nothing did a headline at Politico deem Snowe’s retirement a “boon for Democrats.”

But the state of the race is much more ambiguous than that. One reason is that the field won’t be set until after March 15, the candidate filing deadline, just two weeks after Snow’s announcement. Yet already the Cook Political Report moved the contest a toss-up. “It went from a sure GOP seat to one they’ll have to work for,” says Jennifer Duffy, a senior editor for Cook.

While Mainers prefer Democrats for president, they last elected a Democrat to the Senate in 1988. Registered independents, many of whom are Democratic-leaning, far outnumber members of both parties. Indeed, political independence is something Maine voters take pride in, which may explain why there haven’t been successive governors of the same party in sixty years and why two recent governors were independents.

One of those independent governors, Angus King, announced his candidacy for Snowe’s Senate seat on Monday, a move which has implications for both Republicans and Democrats. A well-known and popular public affairs television host, King served two terms as governor but hasn’t been in office since 2003. Despite his independence, he leans liberal and will likely take away more votes from the Democratic nominee.

Observers say King’s entry complicates the potential candidacy of Congresswoman Chellie Pingree, a progressive Democrat representing Maine’s more liberal First Congressional District. Pingree would have the advantage in a Democratic primary against former governor John Baldacci, who is also considering running but left office last year rather unpopular. Pingree’s problem isn’t popularity with Democrats—she easily won reelection to her House seat in 2010—nor is it with money; her husband is a wealthy hedge fund manager with plenty of cash on hand.

But with King in the race, Pingree may be reluctant to run against him and split the Democratic vote. And Pingree’s well-deserved liberal image may hurt her in a general election, as it did when she ran for Senate and lost badly against Susan Collins in 2002. Pingree may decide she would prefer keeping her safe House seat rather than coming in second or even third place against King and a moderate, Snowe-like Republican nominee.

This is the other unknown variable: the Republican candidate. Snowe was facing a primary challenger from the right, the little-known Scott D’Amboise, but most Maine Republicans don’t see his relevance now that the seat is open. Among the serious Republicans considering getting in are Bill Schneider, the state’s attorney general (who is appointed, not elected); Rick Bennett, the former state senate president and former candidate for Congress; and Charlie Summers, the Maine secretary of state (elected by the state legislature), an Iraq war veteran, and a former candidate for Congress. Of the three, Schneider is believed to have the best chance of earning an endorsement from Snowe herself, and Bennett is considered a gifted retail politician. All three have taken out petitions for running.

The scenario Republicans hope for would be one that mirrors the 2010 gubernatorial race. In that election, Republican Paul LePage benefitted from a split on the left between Democrat Libby Mitchell and liberal independent Eliot Cutler, with Mitchell actually falling to a distant third place. With a moderate in the mold of Snowe or Collins on the ballot, Republicans in 2012 might be able to take advantage of such a divide.

But that’s a big maybe. In the first significant poll of the race since Snowe’s departure, PPP found that King would win a hypothetical three-way race against Pingree and Summers. The poll includes two candidates who aren’t even in the race yet, but time grows short for aspirants to jump in.

Update: WCSH reports that Chellie Pingree will not enter the Senate race and will run for reelection in her House seat. Here's her statement:

After careful consideration I have decided to run for re-election to the U.S. House this year. This has been a very difficult decision and I will always be grateful for the tremendous support I've received from people all across Maine and around the country. I have been humbled by the enthusiastic encouragement I've gotten-from my neighbors here in Maine to my colleagues in Washington.

There is much at stake in this election and although the prospect of running for and possibly serving in the United States Senate was very exciting, in the end I concluded that I will best serve the people of Maine by running for re-election to the House.

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