Former Maine Gov. Angus King (I) declared his candidacy to succeed retiring Sen. Olympia Snowe (R) this evening.

Despite his lack of official party affiliation, state operatives have repeatedly described King as a worthy opponent, more frontrunner than spoiler. His political ideology is described as socially moderate or liberal and fiscally conservative.

And the middle ground is where King is planting his flag.

“In a closely divided Senate, I can be a broker for common sense,” King said in his announcement at Bowdoin College in Brunswick, Maine. “In a national debate that’s largely taken over by the extremes … I can speak for the middle.”

“This is an extremely important election and a lot is at stake, including the control of the U.S. Senate,” Pingree said.

“While I have been humbled by the tremendous outpouring of support encouraging me to run, I’m going to continue thinking very carefully about whether or not I should enter this race, and will take all the factors into consideration before deciding how I can best serve the people of Maine,” she added.

She is expected to decide in the coming days.

While Democrats were ebullient last week over Snowe’s retirement and the prospect of picking up her Senate seat, numerous Maine Democrats expressed apprehension today about the King campaign and the prospect of a three-way race. Ex-Gov. John Baldacci is also taking steps to seek the Democratic nomination.

Republicans mulling the open-seat contest are Maine Secretary of State Charlie Summers, state Treasurer Bruce Poliquin and Attorney General William Schneider.

Roll Call currently rates this race as a Tossup.