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Robert F. Bukaty, Associated Press
U.S. Rep. Chellie Pingree, D-Maine, speaks at a news conference Friday, March 2, 2012, in Portland, Maine. Pingree is considering a run for the Senate seat being vacated by Sen. Olympia Snowe, R-Maine.

PORTLAND, Maine — Democratic U.S. Rep. Chellie Pingree said Wednesday that she decided against running for the U.S. Senate seat being vacated by Republican Olympia Snowe, choosing instead to defend her House seat.

Pingree, who represents Maine's southern coastal district, said it was a difficult decision and that she's grateful for the support she received from Maine and across the country. In the end, she concluded that she can best serve Maine by running for re-election to the House, she said.

"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," she said in a statement.

Snowe, a moderate Republican, announced last week that she wouldn't seek a fourth term, citing frustrations over partisan politics and gridlock in the Senate. Her decision set off a scramble by potential candidates since Republicans and Democrats have only until March 15 to submit 2,000 signatures to get on the June primary ballot.

If Pingree had run for Senate, she would have faced her longtime friend, popular former Gov. Angus King, who announced he's running as an independent. Her decision not to run could mean more Democratic support for King, who if elected would be courted by both parties to join their Senate caucus.

"This is a personal relief to me because I wasn't looking forward to running against a friend," King said in a statement.

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

Republicans suspect 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.

Crystal Canney, King's spokeswoman, said he would have no comment Wednesday.

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.

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 waiting to see how things shake out.

"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 raised more than $300,000 to show Pingree would have national grassroots support in a run for Senate.

Adam Green, co-founder of The Progressive Change Campaign, another group backing Pingree, suggested 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 are already in the race, 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.

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