U.S. Republican presidential nominee Donald Trump told supporters that having a faulty microphone when he debated his Democratic rival Hillary Clinton made it "difficult."

Trump had complained about problems with the microphone since the debate last Monday (September 26), but it wasn't until last night that the Commission on Presidential Debates confirmed that there were problems with Trump's audio, but they said it affected the sound level in the debate hall.

However, the sound was not affected for the record-setting audience of 84 million people who tuned into the 90-minute exchange. An online article on Variety.com reported that Trump's pre-debate stage and sound check was rushed because he arrived 45 minutes late. Variety said the check gives campaign staffers the opportunity to check sound levels at various points in the hall.

At the rally in Novi, Michigan, Trump wondered aloud whether the microphone problem was deliberate.

"It's difficult, and when you have a situation like that and you know it's bad and you think that 100 million people are watching, what do you do, stop the show? I wonder why it was bad," he said.

The latest Reuters/Ipsos national tracking poll, released yesterday, showed Clinton leading by 43 per cent to Trump's 38 per cent among likely voters.