Former Gov. Angus King is holding on to a significantly narrowed lead in Maine’s U.S. Senate race, according to results of two polls released Wednesday.

One poll shows King with a 15-percentage-point lead over Republican Charlie Summers, while the other shows King with an 8-point lead.

King, an independent, had a 25-point lead in the polls in June, before pro-Republican groups aired several waves of anti-King TV ads statewide.

Public Policy Polling, a national pollster, said Wednesday that its poll shows King with 43 percent, Summers with 35 percent and Democrat Cynthia Dill with 14 percent. Eight percent of voters were undecided, and the poll did not ask voters if they supported any of the other candidates in the race.

Public Policy Polling surveyed 804 likely Maine voters using automated telephone interviews on Sept. 17 and 18. The margin of error is 3.5 percentage points, it said.

The Maine People’s Resource Center released a separate poll showing King with 43.8 percent support, Summers with 28.2 percent and Dill with 14.9 percent. About 7 percent said they were undecided and 6.1 percent said they planned to vote for someone else.

There are lesser-known independents in the race — Danny Dalton, Andrew Ian Dodge and Steve Woods.

The Maine People’s Resource Center is a nonprofit research organization affiliated with the Maine People’s Alliance, a liberal advocacy group. Its automated telephone poll surveyed 856 registered Maine voters in likely voter households from Sept. 15 through Sept. 17. The margin of error is 3.35 percentage points, according to the group.

Summers’ campaign trumpeted the numbers in emails to the media and supporters Wednesday.

“While polls are just a snapshot in time, it’s abundantly clear that anyone who thought this race would be a coronation was dead wrong,” Summers’ spokesman, Drew Brandewie, said in a written statement.

The King campaign, meanwhile, said the tightening race is not a surprise, given the advertising attacks.

“We currently have a double-digit lead in one poll and a smaller lead in another — after $1.7 million in negative advertising from out-of-state, right-wing groups — it’s obvious this race can’t be taken for granted and we are not going to,” King spokeswoman Crystal Canney said in a written statement.

Dill released a statement in response to the Maine People’s Resource Center poll, saying it proves that she is winning over voters and that expensive advertising is not helping her opponents.

“The numbers in today’s poll reflect what I am hearing from Maine people,” she said.

The race is being closely watched nationally. Republicans in Washington, D.C., and the U.S. Chamber of Commerce have targeted Maine as one of a small number of states that could help the party win a majority in the Senate. The poll results likely mean the groups will continue to pour money into the race.

“This is now a horse race,” said Rob Engstrom, political director for the U.S. Chamber of Commerce. “We believe this is now a tier-one opportunity.”

The national Republican and Democratic senatorial committees did not immediately respond to Maine’s new poll numbers.

King got into the race as the clear front-runner. A poll done for the Portland Press Herald by Critical Insights in late June indicated that King was favored by 55 percent of surveyed voters, compared with 27 percent for Summers and 7 percent for Dill. Nine percent said they were undecided.

The change since then appears to be the direct result of anti-King television ads that have been airing statewide for several weeks. Outside groups that hope to win a Republican majority in the Senate have so far committed more than $1.5 million for ads criticizing King’s record as governor and his role as a wind power developer.

In addition to criticizing King, one of the Republican-backed ads touts Dill as “a Democrat you can feel good about,” in an effort to move Democratic voters from King to Dill.

While the two polls released Wednesday match in most areas, they have two big discrepancies.

Both have King with about 43 percent support and Dill with about 14 percent. However, the Maine People’s Resource Center has Summers with 28.2 percent support and Public Policy Polling has Summers with 35 percent.

That 6.8-percentage-point discrepancy is just inside the combined margins of error for the two polls — 6.85 percent.

Mike Tipping, lead pollster for the Maine People’s Resource Center, said the difference is likely due to the fact that voters could choose “other” Senate candidates in his poll and not in the Public Policy Polling poll.

The other big difference between the polls is in King’s support among Republican voters.

Both polls show King’s support strongest among Democrats and independents. However, the Maine People’s Resource Center found that King has the support of 23.4 percent of Republicans, while Public Policy Polling says King has the support of just 13 percent of Republicans.

Public Policy Polling even suggested in its analysis of the results that King might decide to publicly declare that he would caucus with Democrats if elected to the Senate, because he no longer has strong Republican support to lose.

“The Maine Senate race is closer now than anyone really expected it to be,” Dean Debnam, president of Public Policy Polling, said in a blog post. “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.”

The Public Policy Polling survey says King’s popularity has dipped along with his lead. Just over half — 52 percent of voters surveyed — expressed a favorable opinion of King, while 35 percent had an unfavorable opinion.

In March, according to the pollster, King had a 62 percent favorable rating and a 24 percent unfavorable rating.

King’s biggest favorability drop was among Republicans, according to Public Policy Polling’s blog post analyzing the results.

“If there’s good news for King in this particular poll, it’s that neither of his opponents (is) exactly setting the world on fire,” the blog post said.

Summers’ favorable-to-unfavorable ratio is 36 to 40, and Dill’s is 24 to 37, according to the poll.

In the race for president, meanwhile, the polls show President Obama holding a solid lead among surveyed Maine voters over Republican Mitt Romney.

The Maine People’s Resource Center shows Obama leading 53.5 percent to 37.3 percent.

Public Policy Polling has Obama leading 55 percent to 39 percent.

Neither poll was paid for by any campaign or political party, the pollsters said.

Although the Maine People’s Resource Center is affiliated with a liberal advocacy group, it has earned credibility with the accuracy of past polling results, including in Portland’s mayoral race last year and the 2010 governor’s race.

Staff Writer John Richardson can be contacted at 791-6324 or at:

jrichardson@pressherald.com