From this article in the National Law Journal:
The Libertarian Party and the Green Party are suing Romney—along with the Republican National Committee, the Democratic National Committee, President Barack Obama and the Commission on Presidential Debates—over the exclusion of third-party candidates from general-election debates.
In a lawsuit filed in September in federal district court in Washington, the plaintiffs claimed that the politicians, the DNC, the RNC and the debate commission violated antitrust laws. The complaint accused the defendants of illegally conspiring to exclude third-party candidates from the “presidential debates market, the presidential campaign market, and the electoral politics market.” Romney and his co-defendants filed papers last week asking U.S. District Judge Rosemary Collyer to dismiss the case. […]
“Somehow running for president isn’t a commercial activity, even though it’s projected that in 2016 races, collectively, people will spend $5 billion to $10 billion,” [plaintiff’s attorney Bruce] Fein said.
Obama and Romney’s campaigns signed an agreement in which they agreed not to debate other candidates, Fein said, a move that went beyond an individual politician making a unilateral, politically motivated decision.
“The antitrust laws are concerned with concerted action,” Fein said.