WASHINGTON — Democratic U.S. Rep. Chellie Pingree said today she won’t run for the U.S. Senate, a decision that appears to boost the candidacy of independent former Gov. Angus King.

In the wake of Pingree’s decision, Democrats might coalesce behind the socially liberal former governor, who won two campaigns in 1994 and 1998 as an independent, the second one with nearly 59 percent of the vote in a five-way race.

Pingree said in an interview today that, in large part, she is not running because of the fear that she and King might divide the Democratic base, thus paving the way for a victory by a Republican contender.

In the aftermath of Republican U.S. Sen. Olympia Snowe’s decision not to seek a fourth term, the Senate race in Maine is considered a key race nationally, one whose outcome could decide whether Democrats retain a Senate majority.

Several Maine and national Democratic officials and strategists interviewed today said it’s possible that many Maine Democrats may decide to back King over a member of their own party. But they stressed that there have been no overtures to King and no party decision to support him.

For his part, King has been adamant that he is running as an independent. He said that if he wins the race, he won’t commit to caucusing in the Senate with either Democrats or Republicans until he goes to Washington.

Pingree, who has represented Maine’s 1st Congressional District since 2008, had considered a run for Senate after Snowe announced her retirement last week.

Pingree’s decision will likely prompt many, if not all, of the 12 Democrats who have expressed interest in succeeding her in the House to drop out of the race.

It remains to be seen which Republicans eyeing her seat will go ahead now that Pingree is seeking re-election. She is seen as the favorite to win re-election as an incumbent in the Democratic-leaning district in southern Maine.

After King announced his independent bid Monday night, Pingree had acknowledged that she shared widely discussed concerns that she and King might divide the Democratic base, thus opening the door to the Senate for a Republican candidate.

Pingree is a member of the House Progressive Caucus, and had been urged to run for the Senate by liberal groups such as Moveon.org.

“What worried me most is that this race could determine the balance of power in the U.S. Senate,” Pingree said today in a phone interview. “We have seen three-way contests before become very complex (in Maine) and I didn’t want to take the chance that my entering the race would make it more likely for a Republican to be the next senator from the state of Maine.”

Pingree’s husband, S. Donald Sussman, a frequent Democratic donor, is buying a 5 percent equity stake in MaineToday Media, which owns The Portland Press Herald/Maine Sunday Telegram, the Kennebec Journal in Augusta, the Morning Sentinel in Waterville and other media outlets.

Pingree had been considered the strongest Democratic candidate for the Senate race after Rep. Mike Michaud, D-2nd, decided against a Senate run. Some Democrats and analysts thought Michaud could be as strong or stronger because he is a moderate from the more conservative northern part of the state.

Now, Pingree’s exit could mean that Democrats at least tacitly throw their support to King, a social liberal who has contributed money to President Obama’s campaigns.

Indeed, the executive director of the National Republican Senatorial Committee in Washington, Rob Jesmer, charged today in a release that “the fix has been in for national Democrats to privately anoint Independent candidate Angus King as their standard-bearer.”

But Pingree said in the interview today that she was not pressured to get out of the race to make way for King.

“No one from the Democratic Party tried to convince me to get out of the race,” Pingree said. “They encouraged me strongly to stay in the race.”

Democratic Party officials have not been talking to King, either, Pingree said.

Pingree said earlier this week, before announcing her decision not to run, that she has talked about the race with King, a close friend who came to her House last fall for Thanksgiving dinner. King gave Pingree advance notice before he made public his decision to run on Monday night.

But Pingree said she and King cut no deals.

“People end up deciding whether or not they want to run for office for the reasons that are important to them and it’s always good to stay in the conversation with people, but it doesn’t mean that everyone gets in a back room, smokes some cigars and a deal is cut,” Pingree said. “It’s just not like that.”

King said in a statement today that he was surprised to learn of Pingree’s decision, adding that, “This is a personal relief to me because I wasn’t looking forward to running against a friend.”

Ben Grant, Maine Democratic Party chairman, said, “the Maine Democratic Party will support the Democrat who is on the ballot, whoever that may be.”

Grant wouldn’t rule out the possibility that Democrats could wind up backing King. But he stressed that “any of that kind of speculation is premature.

“We all know that there is going to be pressure for the Democrats to put their support behind Gov. King,” Grant said. But, “that is not a discussion we are having right now.”

Grant said Democrats are still gathering signatures to get on the ballot, and that the state and national Democratic parties are “taking this as a step by step process” that involves talking with Democrats who are trying to get on the ballot.

Grant said the state party has talked to officials with the national Democratic Senatorial Campaign Committee “about next steps. (But) We haven’t had any discussions with the DSCC about any potential maneuvering with Gov. King.”

Dennis Bailey, a former adviser to King who is not involved with the candidate’s Senate bid, said he would be surprised if King has been a part of any talks with Democrats.

King is “an independent not just in name only,” Bailey said. “I just can’t imagine him doing any kind of backroom talks about it. If there is any maneuvering going on it is only within the Democratic Party.”

But a Maine Democratic insider, who spoke on condition of anonymity because he didn’t want to pre-empt party leaders, said Republican Paul LePage’s narrow win in the 2010 Maine gubernatorial race is fresh on the minds of Maine Democrats. In that race, a relatively weak Democrat, Libby Mitchell, finished a distant third behind LePage and independent Eliot Cutler.

“Many Maine Democrats are unlikely to make the ‘Libby’ mistake a second time and will coalesce around King early,” the Democratic insider said. “Similarly, the Democratic Party’s elected leadership and campaign apparatus will go out of its way to do no harm to Angus. The calculation is that King is preferable to any Republican and is more likely to caucus with Democrats once elected.”

A spokesman for the Democratic Senatorial Campaign Committee declined comment today.

But a Democratic strategist in Washington familiar with the committee, who spoke on condition of anonymity, said the committee has not had any contact with King, either before he announced his independent run or since.

However, national Democrats are considering whether King’s candidacy gives the party its best chance to ensure that a Republican doesn’t retain Snowe’s seat.

“There is no question that if independents and Democrats quickly coalesce behind King that eliminates any chance Republicans have to be competitive in this race,” the Democratic strategist said.

King said in an interview Tuesday with the Portland Press Herald that he would wait until he gets to Washington to decide whether to caucus with Democrats or Republicans and base his decision on “what’s most effective for Maine,” not an ideological preference for either party.

King is a social liberal who has backed same-sex marriage and would seem more likely to align with Democrats, though he also calls himself a fiscal conservative. King gave $1,750 to Barack Obama’s 2008 campaign and another $1,000 last year to Obama’s reelection campaign. King supported George W. Bush in 2000, but endorsed Democrat John Kerry in 2004.

But because his campaign is built on the message that political parties are causing gridlock in Washington, it wouldn’t make sense to announce now which party he would caucus with, King said in the interview Tuesday. He said he expects both Democrats and Republicans would ask him to join their caucuses. If control of the Senate could be influenced by his decision, he said, he would be in a position of power – “the most popular girl at the prom.”

Jennifer Duffy, a senior editor at the nonpartisan Cook Political Report in Washington, said King will come under a lot of pressure during the race to say which party he will align with if elected. But King could tell Maine voters he will simply caucus with whichever party would be in the majority with his vote, “which would afford him a lot of power in the long term,” Duffy said.

Saying he won’t align with either party is not an option because that’s the way a senator receives a committee assignment.

Currently, there are two independent senators, both of them aligned with the Democratic caucus, Bernie Sanders of Vermont and Joe Lieberman of Connecticut.

Duffy noted that Democrats cut a deal in 2006 with Sanders to avoid a three-way race. Sanders, a socialist, was put on the Democratic primary ballot, won the nomination, refused the nomination and then ran as an independent.

“I’m wondering if Democrats are thinking about doing something similar this year,” Duffy said.

A Democratic polling firm, Public Policy Polling, ran a survey March 2-4 of 1,256 Maine voters and found King to be the strongest Senate candidate. King got 36 percent of the vote to Pingree’s 31 percent and 28 percent for Secretary of State Charlie Summers, one of the Republicans considering the Senate race.

The firm did not survey voters on a three-way race involving former Democratic Gov. John Baldacci, who is considering a run for the Senate seat. But the poll did find that in one-on-one match ups with Republicans, Baldacci didn’t do as well as Pingree. While Pingree had an average lead of 16 points against various potential GOP candidates, Baldacci’s lead averaged 8 points.

The firm’s analysis of its survey said that King’s personal favorability rating is very high, 62 percent. If King “can hold his initial support it’s his race to lose,” the analysis said prior to Pingree dropping out.

“Baldacci would be a considerably weaker candidate for the Democrats,” said PPP’s Tom Jensen, in the analysis of the firm’s poll. “Even after a year out of office he remains pretty unpopular with only 37 percent of voters rating him favorably to 52 percent with a negative opinion.”

Dan Cashman, a Baldacci spokesman, said Wednesday that Baldacci is “still weighing his options for a potential run and will have an announcement one way or the other when he is ready.”

Baldacci has been working in Washington at the Pentagon in a contract job studying military health care reform issues. That job ends this month, Cashman said, “and any decision he makes regarding a potential Senate run is independent from his position at the Pentagon.”

The leading Republicans in the race appear to be Summers, former Maine Senate President Rick Bennett, state Attorney General William Schneider, state Treasurer Bruce Poliquin, and state Sen. Debra Plowman of Hampden.

Scott D’Ambose of Lisbon Falls, a tea party affiliated conservative who was challenging Snowe, also remains in the GOP primary race.

Former Maine Secretary of State Matt Dunlap, a Democrat, also says he is running for Senate. And Democratic state Sen. Cynthia Dill of Cape Elizabeth, who shifted from a Senate candidate to a possible candidate for Pingree’s seat when it appeared Pingree might run for the Senate, has indicated she would be back in the Senate race if Pingree remained a House candidate.

Public Policy Polling’s Jensen said that the firm’s poll indicated that, the “Republican candidate pool in Maine is weak,” making it tough to benefit enough from a split between King and even a strong Democratic candidate to pull off a GOP win. There’s also no consensus among Republicans about their preferred candidate, Jensen said.

The Public Policy Polling survey found a potential problem with the strategy of King telling voters he will align with whatever party is in the majority, however.

While 51 percent of his supporters said in the PPP survey that they want King to caucus with Democrats, just 25 percent want him to caucus with Republicans. Independents favor King caucusing with Democrats by 40 percent to 27 percent, the poll indicates.

“King will have a hard time holding onto his early Democratic support without a pledge to caucus with the party if he’s elected to the Senate,” said Dean Debnam, president of Public Policy Polling, in a release Tuesday about the poll.

Party candidates have until March 15 to submit at least 2,000 voter signatures to get on the June 12 primary ballot for the Senate seat. Independents have until June 1 to submit at least 4,000 signatures to get on November’s general election ballot.

MaineToday Media Washington Bureau Chief Jonathan Riskind can be contacted at 791-6280 or at: jriskind@mainetoday.com    Twitter: Twitter.com/MaineTodayDC