AUGUSTA, Maine — Two polls released Wednesday found independent Angus King hanging onto a shrinking lead in the campaign for Maine’s open U.S. Senate seat and Democrat Cynthia Dill doubling her support since the last public survey in June.

But the rate at which King’s lead is shrinking and whether Republican Charlie Summers has made any gains are where the two polls — conducted by the Portland-based Maine People’s Resource Center and the North Carolina firm Public Policy Polling — differ.

The two groups released surveys Wednesday that offer the first independent glimpse in almost three months into how the race for the Senate seat being vacated by Olympia Snowe is shaping up. They also offer insight into what role more than $1.5 million in advertising from outside groups with Republican ties has played in altering voters’ opinions.

The Maine People’s Resource Center survey found King with 44 percent support, Summers with 28 percent and Dill with 15 percent. Some 7 percent remained undecided in the contest, and a combined 6 percent chose independents Steve Woods, Andrew Ian Dodge or Danny Dalton, according to the survey.

Meanwhile, Public Policy Polling put King’s support level at 43 percent, Summers’ at 35 percent and Dill’s at 14 percent. The firm’s poll — which didn’t ask about Woods, Dodge and Dalton — found 8 percent of respondents undecided.

The Maine People’s Resource Center — which is affiliated with the left-leaning Maine People’s Alliance, a statewide advocacy group — surveyed 856 registered Maine voters Sept. 15-17; its survey carries a margin of error of 3.35 percent. Public Policy Polling — which was founded by Democratic pollster Dean Debnam — surveyed 804 likely Maine voters on Monday and Tuesday; its poll has a 3.5 percent margin of error.

While the size of King’s lead is unclear, there’s little doubt the independent former governor has lost support since the last public survey in the contest, conducted June 20-25 by the Portland firm Critical Insights. In that survey, King led his rivals with 55 percent support, compared with 27 percent for Summers and 7 percent for Dill.

Since that June survey, three groups with Republican ties have launched more than $1.5 million in advertising campaigns that largely attack King. While that could explain some of King’s drop in support, what’s unclear is the extent to which Summers has benefited. The Maine People’s Resource Center survey showed Summers’ support level barely budging from the June survey, while Public Policy Polling showed an 8-point gain for Summers, putting him within 8 points of King.

One of the three outside advertising campaigns — from a recently formed group Maine Freedom, whose officers have Republican ties — has encouraged Democrats to choose Dill over King through almost $360,000 in ads. That campaign could explain some of the growth in Dill’s support.

Dill, in a statement, said the latest poll results “reflect what I am hearing from Maine people: Maine voters support my pledge to fight for working families, to help our weakened middle class and to support President Obama’s plan for important new laws like the Affordable Care Act.”

Summers spokesman Drew Brandewie said the polls make it “abundantly clear that anyone who thought this race would be a coronation was dead wrong.” He said the surveys also show that the more Maine voters hear about King’s record as governor and in the wind business, “the more his support shrinks.”

King spokeswoman Crystal Canney attributed the movement in poll numbers to an influx of outside advertising dollars attacking King.

“It’s obvious this race can’t be taken for granted and we are not going to,” she said in an email.

Both surveys do indicate some movement among Democrats away from King.

In the Maine People’s Resource Center survey, King was the choice of 54 percent of Democrats, 52 percent of unenrolled voters and 23 percent of Republicans. In mid-June, a survey conducted by the MassINC Polling Group for the Boston public radio station WBU R had King as the choice of 60 percent of Democrats, 57 percent of independents and 31 percent of Republicans.

Public Policy Polling found King was the choice of 58 percent of Democrats, 13 percent of Republicans and 51 percent of independents. Debnam, the firm’s president, suggested it might be time for King to announce he’s caucusing with Democrats, saying he has more to gain from consolidating Democratic support than trying to attract Republican backing.

Forty-three percent of Public Policy Polling respondents said they wanted King to caucus with Democrats, compared with 30 who favored Republicans and 28 percent who said they weren’t sure.

“The Maine Senate race is closer now than anyone really expected it to be,” Debnam said in a statement. “Angus King is losing more Democrats than he is picking up Republicans and although he remains the favorite, a victory for him is not as inevitable as it used to be.”

Public Policy Polling’s survey indicates the barrage of advertising targeting King could be hurting the former governor’s favorability, especially among Republicans. Some 52 percent of voters have a favorable opinion of King in the latest survey, compared with 35 percent who view him unfavorably. That’s in contrast to a Public Policy Polling survey in March that found King with a 62 percent favorability rating and 24 percent rating him unfavorably.

Among Republicans, 22 percent now view him favorably, compared with 43 percent last March. The percentage of Republicans who view him negatively has increased to 65 percent from 38 percent in March.

Both surveys released Wednesday found President Barack Obama maintaining his edge in Maine over Republican Mitt Romney. The Maine People’s Resource Center put Obama’s lead at 54 percent to 37 percent; Public Policy Polling put it at 55 percent to 39 percent.

In the Resource Center’s survey, however, Obama’s lead is smaller in the state’s 2nd Congressional District, where he holds a 48-41 edge. He leads 59 percent to 34 percent in the 1st District.

The Resource Center poll also asked about Libertarian presidential candidate Gary Johnson, who attracted the support of 2 percent of voters statewide. Green Party candidate Jill Stein is also on the Maine ballot.

If the presidential contest in the 2nd Congressional District gets close, Republicans could start campaigning there aggressively, eyeing it as a potential electoral vote pickup for Romney. Maine is one of two states that can apportion its electoral votes by congressional district, though it has never happened in Maine.

The two surveys released Wednesday are likely among the first in a wave of polls to be released between now and Election Day. Both firms promised more results Thursday, including voter opinions on the same-sex marriage ballot measure and surveys sizing up the races in the state’s two congressional districts.