A public interest group plans to file suit against the Federal Election Commission for failing to take action against Crossroads GPS, a conservative nonprofit group that the agency’s lawyers said should have registered as a political committee in 2010 after spending millions on political ads.

Though the FEC’s general counsel recommended further investigation of Crossroads GPS based on its review of the group’s activities in 2010, the matter was dropped. The six member commission needs a four-member majority to authorize an investigation, but split 3-3, with all three Republican members voting against further action.

Craig Holman, a lobbyist for Public Citizen, a D.C.-based watchdog and advocacy group that complained about the matter in 2010, said Republican commissioners were “disregarding FEC policies.” Holman said the suit would be filed in District Court in a few weeks by Scott Nelson, a senior counsel at Public Citizen, and co-counsel Paul Ryan, a lawyer for the Campaign Legal Center. The legal filing will argue that FEC’s action were “arbitrary, capricious, an abuse of discretion and contrary to the law,” according to a press release.

At issue is whether nonprofit groups like Crossroads GPS, which spent over a hundred million during the 2012 election cycle, need to register as political action committees. While the nonprofits’ lawyers have argued that less than half of their budget in the 2010 cycle went to explicitly political purposes, FEC lawyers considered a broader range of spending, and determined that at least 53% of the group’s spending was political. The general counsel’s report recommended opening an investigation with the power to subpoena documents and witnesses, but the Commission’s deadlock prevented that from happening.

Operating as a nonprofit allows Crossroads GPS to keep their donors list private, but if they registered as a political committee they would be required to disclose their identities.