Ever since Maine's senior senator, Olympia Snowe, announced her decision to retire at the end of this congressional term, the assumption has been that Democrats will benefit from the Republican senator's decision.

The race has always been somewhat quirky. The front-runner from the outset was considered to be the state's former independent governor, Angus King. King has refused to declare which party he would caucus with if elected to the Senate, but conventional wisdom has been that the two-term governor would be more friendly to Democrats than Republicans because he is pro-abortion rights, supports "Obamacare" and has been an outspoken critic of the Citizens United Supreme Court ruling that allowed corporations to give unlimited political donations.

King remains the frontrunner, but recent polling in the state has showed his lead decreasing. Earlier this week, one poll showed his lead within single digits of his Republican opponent, Charlie Summers, a sharp turnabout from the start of the summer when King was ahead by nearly 20 points.

King's slip in the polls presents a problem for Democrats. Republicans have been supporting Summers since he won his party's primary in June. Summers has also had support from outside groups, such as the Chamber of Commerce, which has been running ads in the state on Summers' behalf.

Because of his status as an Independent candidate, King has not had the same type of structural support backing his campaign.

The Democratic Senatorial Campaign Committee has notably stayed out of the race. It has not backed the official Democratic nominee, Cynthia Dill, nor has it expressly backed King, given that King has not expressly sided with them. King hasn't had any backing from outside groups, either.

"What we have seen is that Summers has risen a bit in the polls and King has come down slightly, largely as a result of two different phenomen[a]," said Anthony Corrado, a professor of political science at Colby College in Waterville, Maine.

"First, Summers is now starting to coalesce the Republican vote, and that's largely as a result of advertising conducted by the Chamber and the NRSC," Corrado said. "The other side of this was to bring King down a bit by promoting the Democratic contender, Cynthia Dill, which has solely been done by Maine Freedom PAC, a conservative-oriented PAC which has been running pro-Dill ads as a way to encourage Democrats to look to Dill rather than King."

The result has been a dilemma for Democrats in a state that they had previously considered to be a surefire pick-up for them, and a tough race for a candidate who hadn't run for office in more than a decade. King's second term as governor ended in 2003.

"What's particularly interesting is here you have, one, a Senate race where the Democrat has essentially done no advertising and doesn't have the support of the party, where the Republican has largely been supported by outside groups, and where you have a conservative group actually making positive ads for a Democrat as a way to reduce support for the Independent to give the Republican Party a chance," said Corrado. "That's a dynamic you're not seeing anywhere else."

Equally perplexing is the question of how King should proceed, as well as how the Democratic Party should proceed. For national Democrats, there don't appear to be many options besides hoping that King can turn things around or jumping in and deciding to support Dill. Her polling numbers have climbed somewhat in the past couple of months, but they are still far behind King's and Summers'.

For King, Corrado said, there's another option besides going negative in advertisements against Summers or explicitly stating his plans to side with Democrats, if elected.

"Rather than addressing issues about who are you going to caucus with and rather than launching attack ads, launch positive ads that talk about what he would do and why he's different," said Corrado. "I think that that message will be effective because it would reinforce the general image of King as being more independent and not simply repeating the types of politics that are seen in Washington."