"King's background is as a Democrat," said University of Maine political scientist Mark Brewer. "But there is nothing in his tenure as governor other than his service to Hathaway that indicates he is a strict party line guy."
"I would likely guess [he would caucus with] the Democrats, as he has skewed more liberal over time," said Matthew Gagnon, an experienced Maine Republican strategist. "However, he did endorse George W. Bush in 2000, and flirts with the right on some things. My guess is that if the Republicans took control, or it was 50-50 with the GOP winning the White House, he'd caucus with the Republicans to get a committee chairmanship."
If elected, King would be well-positioned to fill a growing gap in the upper chamber, where the middle is quickly shrinking. In addition to Snowe, Sen. Joseph Lieberman of Connecticut, an independent former Democrat who still caucuses with the party, is retiring at the end of his current term. So is moderate Sen. Ben Nelson, D-Neb. Sen. Scott Brown, R-Mass., the third most liberal GOP member of the Senate, according to National Journal's 2011 vote ratings, is at risk of losing his seat to Democrat Elizabeth Warren.
Eliot Cutler, an independent who came within two points of being elected governor in 2010 is backing King. And he's encouraging supporters to prioritize political independence above other characteristics.
"I want to emphasize the importance of independence," Cutler said in an email to supporters on Tuesday. "There will be those who will argue that gender or party is important, that Maine should send another woman to the Senate or that a Republican or a Democrat can invoke the waning power of the parties on our behalf. I respect those who make those arguments, but I think those arguments are wrong."
Data released Tuesday by automated Democratic pollster Public Policy Polling showed that King is popular, especially with Democrats and independents. Supporters say his personality has as much to do with it as his politics.
"It's no secret that some candidates have a public persona and then you sit down with them one-to-one and they are a different person. That is not true with Angus," said former aide Dennis Bailey.
But before he gets to the Senate, King will have to navigate a multiple candidate field, where his success is far from a sure thing. Democratic Rep. Chellie Pingree, popular with liberals, is still looking at the race. But given King's ability to attract Democratic voters, she may opt not to run.
Republicans would be wise to nominate a candidate who can appeal to moderates, so as not to cede the middle to King. Secretary of State Charles Summers, former state Senate President Rick Bennett and Attorney General Bill Schneider are the names party strategists bring up. Treasurer Bruce Poliquin, on the other hand, is more conservative, and would not be as strong with moderates.
National Republicans are already pouncing on speculation that Democrats could be positioning themselves to woo King.
"The silence by national Democrats in this race, and the fact that there doesn't appear to be any organized effort to collect signatures for a Pingree Senate candidacy, makes clear that some type of backroom deal appears to have already taken place between Harry Reid and Angus King," National Republican Senatorial Committee executive director Rob Jesmer said.
When she announced her retirement, the moderate Snowe decried the partisanship that has seized the Senate as a factor in her decision. King could make a similar argument in his campaign and even cite Snowe in his pitch, whether or not she backs him.
The Senate is losing one independent-minded Maine senator, but it's very possible they'll add another in November.