AUGUSTA, Maine — A national nonprofit organization that gained ballot access in 29 states and attempted to run an online presidential primary has purchased ads supporting independent Senate candidate Angus King, calling the former governor “an independent fighter for Maine.”

Television station records show the group Americans Elect has purchased nearly $500,000 in advertising supporting the independent former governor’s U.S. Senate bid. The organization purchased $344,000 in advertising in the Portland market and spent $151,000 in the Bangor market for advertising spots that started running Friday and will continue through Oct. 25, according to the public records.

Expenditure reports filed with the Federal Election Commission show the group also has paid for $150,000 for polling and research in Maine’s Senate race.

Maine’s Republican Party responded swiftly Friday afternoon, requesting a Federal Election Commission investigation into whether King’s campaign illegally coordinated with Americans Elect, since Eliot Cutler, who ran for governor in 2010 as an independent is listed as both a member of the Americans Elect board of directors and one of nine state chairs for King’s campaign.

Outside groups set up to make unlimited expenditures in hopes of influencing elections aren’t allowed under federal election law to coordinate with candidates’ campaigns.

According to news reports, Americans Elect spent nearly two years qualifying for ballot access in hopes of nominating a third-party presidential candidate through an online nominating convention. Despite raising millions of dollars, the group called off its presidential efforts in May after no prospective candidate met the group’s qualifications: attracting 10,000 clicks of support, with the clicks spread across at least 10 states. Former Louisiana Gov. Buddy Roemer came closest.

Reached Friday, Cutler said he stepped down from the Americans Elect board in June and that he was unaware of the group’s ad buy.

“There was going to be a change in focus to state races, generally speaking,” he said. “It was a very conscious decision that in light of that, the people who were on the board would resign.”

Americans Elect spokeswoman Ileana Wachtel called the Maine GOP’s request for an FEC investigation “a PR stunt that has no merit.”

“The expenditure was made in strict compliance with the law,and the organization firmly believes the complaint is baseless,” she said.

Wachtel said New York Mayor Michael Bloomberg, an independent, and Americans Elect founder Peter Ackerman each contributed $500,000 to the King ad campaign. A third donor, Passport Capital founder John Burbank, contributed $750,000.

King is the only candidate Americans Elect intends to support in this election cycle, Wachtel said.

“At this moment, it’s critical that we elect independent-minded leaders to high office, and Angus King running for the United States Senate is that kind of leader,” she said.

King in June had called on his rivals to pledge to discourage spending in the race by outside groups hoping to influence the election. Independent groups are allowed under federal election law to raise and spend unlimited sums as long as they don’t coordinate their spending with candidates. Republican Charlie Summers rejected the pledge while Democrat Cynthia Dill proposed that the candidates also agree to further campaign spending and contribution limits.

“We didn’t know about this ad from Americans Elect, and we certainly didn’t ask for it,” said King spokeswoman Crystal Canney. King’s proposal to have candidates disavow outside spending “would have taken care of this, but we couldn’t get Charlie Summers to agree to it.”

Americans Elect is registered as a 501(c)(4) nonprofit organization and — unlike candidate campaigns and other political committees registered with the Federal Election Commission — isn’t required to disclose its donors. According to the Internal Revenue Service, 501(c)(4) groups are considered social welfare organizations and can engage in some political activity as long as politics isn’t the group’s primary purpose. ProPublica reported earlier this year, however, that dozens of 501(c)(4) organizations have engaged almost exclusively in political activity, spending millions of dollars on political ads and other election efforts this campaign season.

Americans Elect had been registered under Section 527 of the federal tax code until 2010, when it converted to 501(c)(4) status. Organizations registered under Section 527 are required to disclose their donors.

In its filing with the Federal Election Commission, the Maine GOP said Americans Elect should have registered as a political action committee in order to engage in activity designed to influence the outcome of an election.

But Wachtel said Americans Elect isn’t spending money on King’s behalf illegally because the expenditures are less than 5 percent of the organization’s total budget, and the group’s primary activity isn’t supporting political candidates.

The Americans Elect ad buy comes at a time when polls show the race for Maine’s Senate seat tightening. An internal Republican poll released earlier this week showed King’s lead over Summers shrinking to a four-point advantage.

Groups with Republican ties have spent more than $2 million since late July on TV advertising that targets King for his dealings in the wind business and for his fiscal record as governor. Another spot, sponsored by the recently formed group Maine Freedom, encourages Democrats to choose Dill over King.

While the Democratic Senatorial Campaign Committee hasn’t endorsed Dill in the race, the group recently purchased about $400,000 in advertising targeting Summers. The ad, which started running earlier this week, says Summers “marches with the Washington extremists.”