Tonight’s debates will take place without Green Party candidate Jill Stein and Libertarian candidate Gary Johnson. Neither met a threshold of hitting 15 percent in national polls and thus were not invited to participate.

That threshold was set by the group that puts on the debates, the Commission on Presidential Debates (CPD). Since its inception, the CPD has been staffed by elites from the two major parties. They pick the moderators, choose the format, and set the terms for participation.

The current co-chairs of the CPD are former RNC chair-turned-corporate lobbyist Frank Fahrenkopf, Jr. and former Clinton White House Press Secretary Mike McCurry, who today works at the D.C.-based corporate and political communications firm Public Strategies Washington.

Only once has a CPD-sponsored debate included a third-party candidate. In 1992, George H.W. Bush debate negotiators insisted that Ross Perot be included, believing his presence would harm Bill Clinton’s chances of success. In 1996, Republican felt differently Scott Reed, Bob Dole’s debate negotiator that year, later said, “We went into the debate process with a very specific strategy: We didn’t want Perot in the debates. Nothing else really mattered.”

GOP nominee Donald Trump once favored a third-party presence in the debate — when he belonged to one. When he was considering a Reform Party bid in 2000, Trump called third party exclusion “disgraceful“; Trump now opposes the presence of Green and Libertarian candidates this year.

Watch AJ Plus’s short video on how the CPD keeps third party candidates out of the debates:

Johnson and Stein will both will be live-tweeting their thoughts during the debate.