At The Guardian, Glenn Greenwald refers to the work of historian George Farah:

He described how the two political parties in the 1990s joined forces to wrest control over the presidential debates away from the independent League of Women Voters, which had long resisted the parties' efforts to shield their presidential candidates from genuine surprise or challenge. Now run by the party-controlled Commission on Presidential Debates, these rituals are designed to do little more than " eliminate spontaneity" and "exclude all viable third-party voices". Citing a just-leaked 21-page "memorandum of understanding" secretly negotiated by the two campaigns to govern the rules of the debates, Farah recounted: "We have a private corporation that was created by the Republican and Democratic parties called the Commission on Presidential Debates. It seized control of the presidential debates precisely because the League was independent, precisely because this women's organization had the guts to stand up to the candidates that the major-party candidates had nominated. And instead of making public these contracts and resisting the major-party candidates' manipulations, the commission allows the candidates to negotiate these 21-page contracts that dictate all the fundamental terms of the debates."

Here then, within this one process of structuring the presidential debates, we have every active ingredient that typically defines, and degrades, US democracy. The two parties collude in secret. The have the same interests and goals. Everything is done to ensure that the political process is completely scripted and devoid of any spontaneity or reality. All views that reside outside the narrow confines of the two parties are rigidly excluded. Anyone who might challenge or subvert the two-party duopoly is rendered invisible.