Former Republican presidential candidate Mitt Romney has joined forces with his 2012 rival, President Obama, to fight a lawsuit charging the Commission on Presidential Debates with antitrust violations.

The suit pits the two candidates, the commission and the Republican and Democratic national committees against a coalition of third-party political groups led by the 2012 nominees for the Libertarian Party and the liberal Green Party.

The lawsuit charges that the commission, a private nonprofit jointly run by the two major parties to hash out the presidential debate schedule and rules, is an "illegal conspiracy" that violates the Sherman Anti-Trust Act, a law intended to break up anti-competitive business monopolies. The case, filed with the U.S. District Court for Washington D.C., says the commission "impair[s] competition in the marketplace of ideas."

The commission usually excludes candidates from outside of the two major parties. The last time one was included in a debate was in 1992 when independent candidate H. Ross Perot faced off against then-President George H.W. Bush and then-Democratic nominee Bill Clinton. At the time, Perot was polling in the double digits.

No third-party candidate has polled as well since then. The plaintiffs argue that the exclusion from the debates is part of the reason for that.

"The overall objective was and continues to be the entrenchment (of) market power in the presidential debates market, the presidential campaign market, and the electoral politics market of the two major political parties by exercising duopoly control over presidential and vice presidential debates in general election campaigns for the presidency," the lawsuit contends.

In a response filed on Jan. 8, Romney urged the court to accept arguments made by the other defendants in the case and dismiss the complaint on the grounds that the antitrust law governs private commercial entities and not political activity.

"Presidential debates are a quintessential political, non-commercial activity... The antitrust laws were not intended to regulate non-commercial markets like the 'marketplace of ideas' (even assuming such 'markets' exist as anything more that metaphors). Plaintiffs' claims therefore fail," Romney's brief said.

A spokesman for the commission could not be reached for comment.

Geoffrey Mann, executive director of the International Center for Law and Economics, a nonpartisan think tank, said the case was unusual but that the plaintiffs nevertheless had a shot because courts often have interpreted the antitrust law broadly.

"The short answer is that it is not crazy... The commission is a private entity, not a government one, so it doesn't get immunity," he said, adding, "The question is whether the activity amounts to a restraint of trade." It was hard to tell how a court might come down on that, he said.