The National Organization for Marriage, under fire for its financial contributions to the campaign to repeal marriage equality in Maine, has filed a lawsuit challenging the constitutionality of reporting requirements.

NOM filed the lawsuit in response to an investigation by the Maine Ethics Commission into whether the antigay group was skirting campaign finance laws to prevent disclosing its donors’ identities, according to the Bangor Daily News. NOM has raised more than $500,000 for the effort to repeal the marriage equality law.

“Now, the organization has filed a lawsuit in U.S. District Court in Bangor alleging that Maine’s financial reporting requirements are unconstitutional,” reports the Bangor Daily News. “The lawsuit seeks a court injunction prohibiting the state from enforcing a law that NOM officials claim is being used to harass and intimidate opponents of gay marriage.”

NOM executive director Brian Brown called the reporting requirements an “infringement of free speech.”

The Maine election law requires organizations soliciting more than $5,000 for a ballot question to file disclosure reports with the state.