Stage hand workers adjust the Commission on Presidential Debates logo in this 2008 file photo. PAUL J. RICHARDS/AFP/Getty Images

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.

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"Third parties have played a critical role 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.

"I don't think it's inconceivable at all that a popular candidate who's caught the popular imagination could exceed 15 percent in the poll standings," he said. "Now more than ever, when you have more non-affiliated voters than ever, that possibility exists."

Newton Minow, a member of the CPD and the former chairman of the Federal Communications Commission, added that the threshold keeps the debates from becoming a free for all.

"There are 410 candidates - let me repeat that - 410 candidates for president registered with the Federal Election Commission," he said. "You want to have 410 candidates in the debate?"

Farah dismisses that argument. He says the rules for access should be that in addition to being Constitutionally eligible and on enough state ballots to win the presidency, a candidate have five percent support or have polls show that a majority of Americans want him or her in the debates.

"You're not going to get hundreds of candidates on the stage if you use these criteria," he said. Under his proposed system, Farah said, Ross Perot would have participated in the debates in 1996 (as well as 1992), and Ralph Nader and Pat Buchanan would have participated in 2000. Only the two major party candidates would have been on the ballot in 2004 and 2008. "It's not exactly a zoo," he said.

Buchanan, who finished in fourth place in the 2000 presidential race and won four states when he ran in the Republican primary in 1992, told CBS News that the system as it now stands amounts to a "monopoly" maintained by the major parties.

"It's an instrument of the two political parties to ensure that the presidency is passed back and forth between them," he said. "The very fact that this duopoly can keep you out of the debates means you don't play in the Super Bowl."