The presidential and vice presidential debates are sponsored by the Commission on Presidential Debates, a nonprofit corporation that mandates that a candidate have at least 15 percent support in national polls to participate. Since the CPD took over running the debates in 1988, only once has a third party candidate been allowed to participate: In 1992, when Ross Perot joined Bill Clinton and George H.W. Bush on the debate stage.

The dominance of the two major parties at the debates has critics charging that the system is effectively rigged to shut out other voices. Gary Johnson, the Libertarian Party nominee for president and former New Mexico governor, has sued on anti-trust grounds to be included this year. The CPD, he said in an interview, is designed "to protect the interests of Republicans and Democrats."

George Farah, the author of "No Debate: How the Republican and Democratic Parties Secretly Control the Presidential Debates" and the executive director of Open Debates, calls the 15 percent criteria "absurdly high," noting that candidates who reach five percent support qualify for public funding if they reached five percent support in past elections.

"Third parties have played critical issues in raising issues that are critical to the conversation in this country," he said, pointing to the abolition of slavery and the creation of Social Security and public schools. "When you exclude them from the debate, you have a sort of ideological containment."

A little history: The first televised presidential debates, between John F. Kennedy and Richard Nixon, came in 1960. They were sponsored by the major networks, including CBS News. Presidential debates were next held in 1976, with the League of Women Voters in charge; they sponsored the next two sets of debates as well, before the CPD took over in 1988.

Critics of the current system point to the League of Women Voters as an ideal debate moderator, arguing that it was more willing than the CPD to stand up to the parties. In 1980, Jimmy Carter strongly opposed the inclusion of independent candidate John Anderson, who had polled as high as 26 percent, in the debates. (Anderson would ultimately finish with less than 7 percent of the vote.) The League decided to include Anderson anyway, prompting Carter to drop out of the debates and leaving Anderson alone to debate Ronald Reagan. In 1984, the League held a press conference lambasting Reagan and Walter Mondale for rejecting dozens of potential debate moderators.

In an interview, Anderson said the debates are now "pretty well locked into the maintenance of a two-party system."

"Very clearly, the present system is wrong in my humble judgment in that it excludes the possibility that there could rise up a reasonable and probably candidate from someone other than one of the major parties," he said.

In addition to meeting the 15 percent threshold "as determined by five selected national public opinion polling organizations," the CPD mandates that candidates be Constitutionally eligible to be president and be on the ballot in enough states that it is mathematically possible to win the presidency. Alan Schroeder, a professor at Northeastern University and an expert on presidential debates, says the 15 percent threshold is reasonable.