WASHINGTON — A poll released today by Public Policy Polling, a national Democratic firm, found that independent former Gov. Angus King would narrowly win a three-way race against Democratic Congresswoman Chellie Pingree and Republican Secretary of State Charlie Summers.

King garners 36 percent of the vote to Pingree’s 31 percent and Summers’ 28 percent because he pulls away 35 percent of Democratic voters from Pingree and garners 53 percent of the vote from independents, compared to 25 percent of the GOP vote, PPP’s poll of 1,256 Maine voters found.

But King might have to pledge to caucus with Senate Democrats to retain that type of support, according to the PPP poll.

While 51 percent of his supporters want him to caucus with Democrats, just 25 percent want him to caucus with Republicans. Independents favor King caucusing with Democrats by a 40-27 percent margin, the poll found.

“Angus King and Chellie Pingree look like the early favorites in Maine,” said Dean Debnam, President of Public Policy Polling, in a statement. “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.”

The poll included 550 usual Democratic primary voters and 369 usual Republican primary voters. It was conducted March 2-4 and had a margin of error of 2.8 percent for the entire survey, 4.2 percent for the Democratic sample and 5.1 percent for the GOP sample.

The poll, done by automated phone interviews, was not authorized or paid for by any campaign or political party, PPP says.

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.