By Jason McLure

(Reuters) - Former Maine Governor Angus King has announced he'll run as an independent for the U.S. Senate seat vacated by retiring Republican Olympia Snowe, complicating Senate Democrats' efforts to defend their majority in November.

A poll released on Tuesday by the Democratic aligned Public Policy Polling showed King, who announced his bid late Monday, with a favorability rating of 62 percent, the highest of any of the potential candidates who have started gathering signatures to get on the ballot.

The poll also showed King, 67, an independent who governed the state from 1995 to 2003, winning a three-way contest against potential Democratic and Republican candidates.

"He's one of the most popular politicians in the state," says Jim Melcher, a political science professor at the University of Maine Farmington. "Certainly there's a large number of people in Maine who remember him fondly and I think his theme of getting Washington to work together echoes well."

In announcing his candidacy, King vowed to fix the "total dysfuntionality" of Congress. While not saying which party he would align himself with in Washington, he has decried the polarization of Washington politics, which Snowe, a moderate, cited as the reason she is stepping down.

Snowe's February 28 retirement announcement raised hopes among Democrats that they could win her seat in November in an effort to defend their 53-47 majority, which includes two independents that caucus with the Democrats, Joseph Lieberman of Connecticut and Bernie Sanders of Vermont.

"The public is absolutely fed up with what's going on down there," King told reporters Monday night. "By being an independent I don't have to answer to anybody except the people of Maine."

Among the parade of potential candidates eyeing Snowe's Senate spot, although not officially committed, is U.S. Representative Chellie Pingree, a liberal Democrat who is married to hedge fund manager Donald Sussman and whose own House seat is considered hers to give up. Another Democrat challenger is King's successor, former Democratic Governor John Baldacci, who won two statewide races but left office last year with low approval ratings and a stagnant economy.

On the Republican side, Secretary of State Charles Summers Jr., Attorney General William Schneider and Treasurer Bruce Poliquin are all said to be considering bids, though all three hold appointed posts and none of the three has won an election for statewide office.

"The GOP was caught flat-footed," says Ronald Schmidt, a political science professor at the University of Southern Maine. "They don't have an obvious person to run who would be particularly strong."

Analysts say King would take more votes from Democratic-leaning voters than Republicans, which could provide a boost to a conservative Republican candidate in a three-way election. That scenario happened in the 2010 Maine gubernatorial race, in which Tea Party Republican Paul LePage narrowly defeated Independent Eliot Cutler in a three party race.

King lived in Maine since leaving office nine years ago and is co-founder of Independence Wind, an energy company seeking to build a 50-megawatt wind project in western Maine. Trained as a lawyer, he is also a member of a small merger and acquisitions firm.

King's gubernatorial legacy includes a strong economy and a 2002 law that provided a laptop to every middle school student in the state, says Mark Brewer, a political scientist at the University of Maine Orono.

"He's given a lot of credit for some pretty good economic times," said Brewer. "I don't think there's any doubt about it. As of right now you have to make him the preliminary favorite."

King, whose political career began as a legislative assistant to Democratic Senator William Hathaway in the 1970s, endorsed Republican George W. Bush for president in 2000 but backed Democrats John Kerry and Barack Obama in the last two presidential polls.

Still the former governor's efforts to avoid aligning himself with either party are likely to come under scrutiny during the campaign, as voters will not want to be surprised about whom he chooses to vote with in Washington.

(Editing By Barbara Goldberg and Paul Thomasch)