President Barack Obama and Republican presidential candidate Mitt Romney speak at different campaign events in this photo illlustration. - 

The first presidential debate of this election cycle will be held in Denver next week. President Barack Obama and Gov. Mitt Romney will cover a host of domestic issues -- from health care to the economy. Jim Lehrer of PBS NewsHour will moderate.

Here's a short list of who you won't see on stage: Gary Johnson, Jill Stein or Virgil Goode, the most prominent third-party candidates this election cycle.

And as it turns out, our friends at Gallup help decide who's in and who's out of the debates. One criteria for a candidate to get an invite: score an average of 15 percent on five national polls. Gallup editor-in-chief Frank Newport is the one who looks at the polls and reports back to the Commission on Presidential Debates.

The last third-party candidate who qualified by Gallup's measures was Ross Perot in 1992. Perot went on to win 19 percent of the popular vote in his race against then Gov. Bill Clinton and then President George H.W. Bush. Perot's campaign focused largely on economic issues. At that time, the bad economy was a focus of Perot's campaign. He emphasized the high federal deficit and debt, according to Newport.

Plus, "here's an economic element to that race for you -- he had a billion dollars! And was able to spend about $100 million of it on his own race," said Newport.

Just as in 1992, the bad economy is a focus of this election cycle. But that hasn't been enough to challenge the current system.

"There's not a groundswell for third-party candidates this year," said Newport. In a Gallup poll that named Johnson, Stein and Goode alongside Romney and Obama, the third-party candidates each scored close to 1 percent.

One challenger who could have potentially had a Perot-sized impact: New York City Mayor Michael Bloomberg.