U.S. Rep. Chellie Pingree on Wednesday became the second big-name Democrat to decide against running for a U.S. Senate seat that's being vacated by Republican Olympia Snowe, potentially handing over some Democratic support to the candidacy of her longtime friend, former independent Gov. Angus King.

Pingree, who will defend her House seat instead, opted out of a Senate race in which she and King, a former Democrat, could have divided the Democratic vote, giving an advantage to Republicans.

"There is much at stake in this election, and in the end I had to put the best interests of the state and the country ahead of my own," Pingree said Wednesday in a statement.

Snowe announced last week that she wouldn't seek a fourth term, setting off a scramble since candidates have only until March 15 to submit 2,000 signatures to get on the primary ballot.

Her announcement gave Democrats hope of picking off a seat that was expected to stay in Republican hands. Then King shook up the race by announcing his candidacy on Monday. Pingree joined Maine's other congressman, Democratic Rep. Mike Michaud, in deciding to stay out of the race.

National Republicans, without offering specific proof, responded Wednesday by accusing top Democrats in Washington of pushing aside Pingree, who has strong support from progressives, in favor of King, who is a popular figure after serving two terms as governor from 1995 to 2003.

Republicans suspected Democratic leaders may have won some type of assurance from King that he would align himself with the party.

"The decision by national Democrats to throw Chellie Pingree and other proud Democratic leaders in Maine aside, in favor of an `independent' who supported President Bush in 2000, makes clear they are more concerned with holding onto power in Washington, than trying to advance their own party's principles," Rob Jesmer, executive director of the National Republican Senatorial Committee, said in a statement.

If King wins Maine's Senate seat running as an independent, he could either caucus with Democrats or Republicans, or remain independent. Sens. Joe Lieberman of Connecticut and Bernie Sanders of Vermont are independents who caucus with Democrats. King suggested he could caucus on either side of the aisle, depending on the topic.

"He will be caucusing where he feels he'll be most effective," said spokeswoman Crystal Canney, who denied that King was cutting deals. "There are absolutely no behind-the-scenes deals with anyone, or any committee, or any party, at all."

The top Senate Democrat has denied having any discussions with King.

Senate Democratic leader Harry Reid of Nevada was asked at the Capitol if he had spoken at all with King. "No, I've never spoken to anyone named `Angus,'" he told reporters.

Harold Pachios, a lawyer and prominent Democrat in Maine, dismissed the theory that the Democratic machine was pulling strings, saying both Democratic and Republican operatives are watching from the sidelines.

"You have to wait for things to play out. You cannot decide this election in the first two days. It is now early March. The election is in November. Anybody who makes conclusive statements about what's going to happen now doesn't know what they're talking about," Pachios said.

Pingree is popular with Democrats.

MoveOn.org members pledged more than $300,000 to show that Pingree would have national grassroots support in a run for Senate.

Adam Green, co-founder of The Progressive Change Campaign, another group backing Pingree, said Pingree was swayed by the possibility that King would create a three-way race like the one in which Republican Gov. Paul LePage was elected with tea party support in 2010.

"Angus King was willing to create a three-way race that handed this Senate seat to a far-right Republican _ a precise repeat of what happened in Maine's 2010 three-way gubernatorial race," Green said.

One possible Democratic candidate for Snowe's seat is former two-term Gov. John Baldacci, who is collecting signatures but has yet to make a final decision, said spokesman Dan Cashman. Baldacci's job at the Pentagon, where he was hired on a one-year basis to be director of the Department of Defense's Military Health Care Reform Initiative, ends this month. Four other Democrats previously announced they were running for Senate, but some said they'd consider stepping aside if Pingree chose to run.

On the GOP side, previously announced candidate Scott D'Amboise now faces the prospect of four or more GOP competitors, including former state Sen. Rick Bennett. They could be joined additional candidates including Secretary of State Charles Summers, Attorney General William Schneider and state Treasurer Bruce Poliquin.

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Associated Press writers Andrew Miga in Washington, D.C, and Clarke Canfield in Portland, Maine, contributed to this report.