From this article in the National Law Journal:
… Read more ...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.