With just 15 days left until the scheduled Sept. 10 presidential debate between Vice President Kamala Harris and former President Donald Trump, negotiations between their two campaigns have hit an impasse over whether the candidates’ microphones will be muted when it is not their turn to speak, according to four people familiar with the issue.

In June, President Joe Biden’s campaign came to an agreement with Trump’s: There would be two debates — CNN’s on June 27 and ABC’s on Sept. 10 — conducted by mutually negotiated rules. One of the Biden team’s demands — which the Trump team agreed to — was that microphones “will be muted throughout the debate except for the candidate whose turn it is to speak,” as CNN announced on June 15.

“We have told ABC and other networks seeking to host a possible October debate that we believe both candidates’ mics should be live throughout the full broadcast,” Brian Fallon, the Harris campaign’s senior adviser for communications, tells POLITICO. “Our understanding is that Trump’s handlers prefer the muted microphone because they don’t think their candidate can act presidential for 90 minutes on his own. We suspect Trump’s team has not even told their boss about this dispute because it would be too embarrassing to admit they don’t think he can handle himself against Vice President Harris without the benefit of a mute button.”