A liberal group critical of Sen. Susan Collins, R-Maine, is the target of an ethics complaint asking the IRS to investigate its tax-exempt status.

The D.C.-based nonprofit Foundation for Accountability for Civic Trust (FACT) sent a letter to the IRS on Wednesday alleging that Maine Momentum had engaged in a "consistent pattern of activities" focused on Collins, indicating it was working for the "private benefit" of Maine's Democratic entities rather than promoting social welfare, as required by its 501c(4) status. The tax-exempt status also bars organizations from overtly supporting or opposing any particular candidate.

“These organizations are supposed to promote the social welfare of the general public not engage in overtly partisan politics, and there are many facts in this case which bring into question Maine Momentum’s execution of this requirement,” FACT Executive Director Kendra Arnold said in a statement released Friday.

Controversy over groups' 501c(4) status is nothing new. After the Supreme Court's decision in Citizen's United v. FEC, the status has been increasingly used by nonprofits to influence political issues while refusing to disclose corporate donors. As FACT points out, the status precludes any organization "that primarily benefits a private group of citizens." IRS guidance on the 501(c)4 status forbids "direct or indirect participation or intervention in political campaigns on behalf of or in opposition to any candidate for public office." Organizations are able to "engage in some political activities, so long as that is not its primary activity."

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Maine Momentum did not immediately respond to Fox News' request for comment, nor did Collins' office.

Collins' 2020 re-election bid gained attention early on after she voted to confirm Justice Brett Kavanaugh to the Supreme Court in the wake of sexual misconduct allegations against the federal judge. Collins' vote provoked outrage from progressives. At one point during the confirmation process, a group was accused of bribing Collins after it offered her more than $1 million in campaign donations if she voted against Kavanaugh.

FACT's complaint points out ties between Maine Momentum and the state's Democratic Party and cites the group's own mission statement. According to FACT, Maine Momentum paid for dozens of Facebook ads targeting Collins and reserved more than $1 million in air time on Maine television.

The group's website features a photo with several protest signs directed specifically at Collins. This past summer, a Maine Public Radio report cast doubt on the group's purpose, claiming it had used "exploited advocacy rules" to target the Republican senator.

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Among its other work, Maine Momentum has developed a campaign called The 16 Counties Coalition which reportedly focuses on criticizing Collins for her vote in favor of President Trump's tax reform bill.

The 16 Counties Coalition is run by two former Democratic spokespeople, including Willy Ritch, who previously worked for Rep. Chellie Pingree, D-Maine. Chris Glynn, who previously worked for the Maine Democratic Party and state House Speaker Sara Gideon, also led the organization.

The coalition's website specifically targets the "Republican-controlled Senate's policies, which reward wealthy donors at our expense." But Ritch has maintained that the group is limited to education and "accountability." "We're not going to tell anybody who they should or shouldn’t vote for. We’ll leave that fight to other people," Ritch told Maine Public Radio.

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According to FACT, the vast majority of the campaign's social media activity has focused on Collins. "[R]oughly 85% of the tweets by the 16 Counties Coalition have featured attacks on Susan Collins. Indeed, the only account that the 16 Counties Coalition's Twitter handle follows is Susan Collins' Twitter account." The complaint also claims that the group's Facebook page has "never once mentioned another elected official, including when discussing the policy issues that Maine Momentum and 16 Counties claim are their chief priorities."

"Instead, nearly every single post attacks Senator Collins and many focus on the source of her political campaign contributions," the complaint reads.