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

Pingree said she will instead run for a third term in the House 1st District, giving other Democrats a shot at the Senate seat that Republican Olympia Snowe will vacate in the fall.

However, Democrats and Republicans alike face a formidable candidate in King, a social liberal and self-described fiscal conservative who won gubernatorial campaigns in 1994 and 1998. In his re-election bid, King captured nearly 59 percent of the vote in a five-way race.

Snowe’s decision not to seek a fourth term has transformed the Senate race in Maine into a key national contest, the result of which could decide whether Democrats retain a Senate majority. As a result, the race will attract the attention of national party leaders and set new campaign spending records for a federal election in Maine.

Pingree said in an interview Wednesday that she was concerned that the race could determine the balance of power in the Senate. Many observers believe King would capture many Democratic votes, giving a Republican candidate a clean shot at Snowe’s seat.

“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,” she said.

King said in a prepared statement 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.”

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

Pingree’s decision leaves another former governor, John Baldacci, as the most noteworthy prospective Democratic candidate for Snowe’s seat. 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 at the Pentagon studying military health care reform issues. That contract job ends this month, Cashman said, “and any decision he makes regarding a potential Senate run is independent from his position at the Pentagon.”

Former Maine Secretary of State Matt Dunlap, a Democrat, also said he is running for Senate. Democratic state Sen. Cynthia Dill of Cape Elizabeth, who shifted from a Senate candidate to a House candidate when it appeared Pingree might run for the Senate, said Wednesday she is now back in the Senate race.

The Republican field includes about a dozen declared candidates. One of them, state Sen. Debra Plowman of Hampden, said that if she were the GOP nominee, her chances of winning a three-way race in November would have been better if Pingree was the Democratic candidate.

“I would have preferred that she stay in the race, to tell you the truth,” said Plowman, the assistant majority leader. “That would have been much better.”

Several of her potential primary opponents are better known statewide, including former Maine Senate President Rick Bennett, Secretary of State Charles Summers, Attorney General William Schneider and Treasurer Bruce Poliquin.

The GOP field also includes Scott D’Amboise, a businessman from Lisbon Falls who is affiliated with the tea party and began running for the nomination last year.

Summers, 52, said he is committed to running for the Senate and is now focused on gathering the required 2,000 certified signatures by the March 15 deadline.

“I believe I will run, absolutely,” he said.

Bennett, who lives in Oxford and is chief executive officer of a Portland investment research firm, said he is seriously considering running.

Pingree’s decision not to go for the Senate seat 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 for the 1st District seat.

She is seen as the favorite to win re-election as an incumbent in the Democratic-leaning district.

However, a potential opponent, Republican Sen. Jonathan Courtney, the majority leader in the Maine Senate, said Congress is extremely unpopular now and people are looking for a change.

Courtney, 45, lives in Sanford and owns a dry cleaning business in the former mill town. He said that Pingree, who is married to S. Donald Sussman, a wealthy hedge fund manager, is out of out touch with ordinary people.

“We need someone who listens to them and knows what it means to struggle to earn a living,” he said.

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.

Courtney began collecting signatures last week. He said Pingree’s decision to defend her seat won’t change his mind about running for Congress.

Other GOP candidates in the 1st District primary include Markham Gartley, a former secretary of state, and four newcomers with little or no political experience.

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.

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.

Staff Writer Tom Bell contributed to this story.

Jonathan Riskind — 791-6280

[email protected]