"Whatever white-knight or unaligned candidate decides to enter the race, and whomever my fellow Democrats decide is the best person to win this seat, I am the only Democrat in this race to have voted for marriage equality, to have helped expand economic opportunity statewide via expansion of broadband Internet, and to have won an election in the post-Paul LePage era," Dill said.
Republicans: For Republicans to have a chance in the Democratic-leaning state (President Obama won 58 percent there in 2008) they'll need to nominate someone who can appeal to the middle and won't be written off as too partisan.
The three biggest names -- all of whom have taken out papers -- are Secretary of State Charlie Summers, Attorney General William Schneider, and former state Senate President Richard Bennett. All could pull votes from the middle.
Treasurer Bruce Poliquin is also running. He's a more conservative candidate and an ally of tea party-backed Republican Gov. Paul LePage. His political profile has some strategists concerned that he would have trouble with moderates in the general election.
Independents: King, who remains popular over nearly a decade after he left office is the early favorite to win Snowe's seat. The only other independent who would have been formidable is Eliot Cutler, but he's backing King.