“This is America. Everyone is free to donate to the candidate of their choosing, everyone is free to support the candidate of their choosing, but an organization that is not nonpartisan shouldn’t pretend to be so,” Trump campaign manager Bill Stepien told reporters on a conference call.

Stepien called out more than a half-dozen members of the commission, outlining past comments critical of the president and donations to Democratic candidates.

“Listen to their words, read their comments,” Stepien said. “The facts are the facts. I hope — I sincerely hope — these members leave their partisan, anti-Trump beliefs words and actions at the door as we take the next steps forward to the lead-up to the debate in Miami in just a few weeks.”

Trump senior adviser Jason Miller called commission members “permanent swamp monsters” and likened them to Biden.

The commission did not respond to a request for comment.

The group is overseen by Frank Fahrenkopf, who served as chairman of the Republican National Committee under former President Ronald Reagan. The organization’s leadership also includes Dorothy Ridings, a past president of the League of Women Voters, and Kenneth Wollack, a former president of the National Democratic Institute. The organization was co-founded by Fahrenkopf and Paul Kirk, a former Democratic National Committee chairman.

On its website, the Commission on Presidential Debates describes itself as “an independent organization” which is “not controlled by any political party or outside organization” and doesn’t “endorse, support, or oppose political candidates or parties.”

Trump previewed the campaign’s message earlier Thursday, tweeting: “Why would I allow the Debate Commission to change the rules for the second and third Debates when I easily won last time?”

Yet for all their criticism, Trump advisers made it clear that the president wouldn’t back down from future forums. Miller said Trump “fully plans on participating and winning both the second and third debates.”

During the call, Max Miller, a senior Trump campaign official who is overseeing the debate negotiations, said the Biden team had suggested several changes. They included a limitation on open discussion to add greater structure and the use of a mute button on a candidate’s microphone.

Biden spokesperson Andrew Bates denied the Democratic campaign had suggested a mute button, adding that “since June, we have consistently said we would participate in all three debates, under rules that the CPD established.”

“Perhaps if the President and his team spent as much time worrying about the country as they do about debate rules, he would not be trailing and would not have engaged in the desperate, out-of-control behavior — emblematic of his tenure — that he showed on Tuesday night,” Bates said. “Our position is clear: we will participate under the CPD's rules. The only real question left is whether the President will start following the rules in the next two debates.”

Trump has been complaining about the commission for months. In December, Trump tweeted that the group was “very biased.”

In 2016, Trump blamed his lackluster performance in the first debate on a “defective” microphone. The commission later released a statement conceding “there were issues regarding Donald Trump’s audio that affected the sound level in the debate hall."