PORTLAND, Maine — Independent Angus King leads the race to fill U.S. Sen. Olympia Snowe’s soon-to-be-empty seat with nearly double the support of his closest competitor and four months of campaigning to go before the November vote, according to a poll by Portland-based Critical Insights.

King, a former two-term governor, was favored by 55 percent of surveyed voters, compared with 27 percent support for Snowe’s fellow Republican, Charlie Summers, and 7 percent for Democrat Cynthia Dill, according to the poll. Nine percent were undecided. The poll reflects earlier polls that reported similar results.

Snowe took the political establishment by surprise when she announced Feb. 28 that she wouldn’t seek a fourth term, citing partisan polarization and political gridlock. Summers and Dill emerged as victors in their respective primaries which together drew 10 Republicans and Democrats.

The survey, commissioned by The Portland Press Herald, indicated King was drawing support from both parties, especially Democrats. Sixty-seven percent of Democrats said they support King. Dill, a state senator from Cape Elizabeth, drew support from only 17 percent of Democrats surveyed in the poll.

Critical Insights surveyed 615 Maine voters from June 20 to 25, after Summers’ and Dill’s primary victories. The margin of error was 4 percentage points on statewide polling. The margin of error was 5.75 in the 1st Congressional District and 5.4 percentage points in the 2nd District.

The poll had Democratic U.S. Reps. Chellie Pingree and Michael Michaud leading Republican challengers Jon Courtney and Kevin Raye in the congressional races.

In the 1st District, Pingree led Courtney 57 percent to 31 percent among those polled. Michaud led Raye 47 percent to 35 percent in the 2nd District, where a large number of respondents were still undecided, the survey said.

In the presidential race, the poll showed Mainers favoring President Barack Obama over Republican challenger Mitt Romney, 49 percent to 35 percent, while 13 percent were undecided.

Obama had strong support from Maine women, and voters younger than 35 and older than 55. Romney’s support was strongest among middle-age voters and men, according to the poll.