The U.S. Commission on Presidential Debates on Monday released the polls it will use to determine which candidates will take to the stage in September for their first presidential debate—and shut out third-party candidates in the process.

The Hill reports:

Candidates will need to hit an average of 15 percent in polls conducted by ABC/Washington Post, CBS/New York Times, CNN/Opinion Research Corporation, Fox News, and NBC/Wall Street Journal. The 15 percent threshold had been announced months ago, but the commission released its polling selections on Monday after consultation with Frank Newport, the editor-in-chief of Gallup.

As of Monday, neither Libertarian Gary Johnson nor the Green Party's Jill Stein had enough support to get a spot onstage alongside Republican nominee Donald Trump and Democratic nominee Hillary Clinton, which open debate advocates say amounts to a fraud of bipartisanism.

One such advocacy group, RootsAction, launched a petition on Monday calling for the executives at ABC, CBS, NBC, Fox Broadcasting, PBS, CNN, Fox News, MSNBC, Univision, and Telemundo to present debates including all four candidates, even if the commission—or Trump or Clinton—wants otherwise.

"If Trump or Clinton balk, let them know you're happy to leave their podium empty," the petition states.

RootsAction co-founder Jeff Cohen also wrote a column on Monday that noted:

If ten major TV networks got together and decided to nationally televise a presidential debate restricted to Republican nominee Donald Trump and right-leaning Libertarian nominee Gary Johnson, while barring other candidates including Democrat Hillary Clinton, it would be recognized as an act of media bias or exclusion. But what if the televised debates this fall are restricted to just Trump and Clinton? That, too, needs to be recognized as an intentional act of media exclusion.

In addition to the importance of highlighting strong third-party candidates, Cohen said, allowing Stein and Johnson to debate would reflect the growing public disillusionment with establishment politics.

"In the political realm, after Ds and Rs unabashedly announced that they formed a commission for the purpose of maintaining their duopoly of power, one might expect a reaction from TV news executives—especially in an election year when the D and R nominees are so widely disliked and mistrusted," he wrote.

Earlier this month, a federal judge rejected a lawsuit by the Green and Libertarian Parties that sought to claim a podium at the debates.

"Plaintiffs in this case have not alleged a non-speculative injury traceable to the Commission," U.S. District Court Judge Rosemary Collyer wrote at the time.

Johnson's campaign manager Ron Nielson expressed disappointment in that rationale, stating, "There is clearly an unprecedented desire for alternatives to the Republican and Democratic nominees, and voters deserve an opportunity to see and hear that there are, in fact, other credible, serious choices."