Pressed by NBC anchor Savannah Guthrie, Trump refused to disavow the far-right, anti-Semitic QAnon conspiracy theory.
"I'll tell you what I do know about, I know about Antifa and I know about the radical left", said Trump, claiming the groups are "burning down our cities".
The second presidential debate had been scheduled for Thursday, but Trump pulled out after organizers said it would be virtual to lessen the risk of infection.
The president faced a series of tough questions Thursday night regarding his health, coronavirus leadership, winking at QAnon conspiracy theorists, the reporting on his taxes, and many other pressing topics.
The FBI in 2019 labelled the group a domestic terror threat, but on Thursday Trump claimed to "know nothing about QAnon". Savannah Guthrie: "I just told you".
Trump replied, saying: "I know nothing about it".
Back in August, Trump again claimed to not know anything about the group but added that they "like me very much, which I appreciate".
"The President's unwillingness to denounce an absurd and risky conspiracy theory last night continues an alarming pattern: politicians and parties refuse to forcefully and convincingly repudiate groups like antifa, white supremacists, and conspiracy peddlers", said Romney.
"I don't get that", she said.
One Twitter user wrote: "This Trump town hall is a f***ing disaster for him".
"As the parties rush down a rabbit hole, they may be opening a door to political movement that could eventually eclipse them both", he warned.
Despite its wildly xenophobic rhetoric and Federal Bureau of Investigation threat designation, the group has gained more traction in recent months, with several top GOP figures expressing support, including Trump, his first national security adviser Michael Flynn and many congressional candidates.
Guthrie pressed Trump bringing up the fact that he retweeted a baseless QAnon conspiracy theory that former President Barack Obama had Navy SEALs killed over some odd plot involving an Osama bin Laden body double. "People can decide for themselves". "You're the President. You're not like someone's insane uncle who can retweet whatever".