The stunning moment came when moderator Chris Wallace asked Trump if he was ready to condemn White supremacists and say they need to stand down during ongoing demonstrations across the country.
"Sure, I'm willing to (tell them to stand down), but I would say almost everything I see is from the left wing, not from the right wing. I'm willing to do anything. I want to see peace," Trump said.
"Who would you like me to condemn?" Trump asked Wallace. Biden could be heard twice saying, "Proud Boys."
Trump continued: "Proud Boys -- stand back and stand by. But I'll tell you what. I'll tell you what. Somebody's got to do something about Antifa and the left because this is not a right wing problem."
The President's pointed refusal to denounce White supremacists and his mention of the group, specifically, drew immediate celebration from members of the Proud Boys. Images of an updated Proud Boys logo featuring the President's "stand by" remark in the group's signature yellow and black swiftly circulated online.
Asked about Trump's remarks after the debate Tuesday, Democratic vice presidential nominee Kamala Harris told CNN's Jake Tapper, "I heard what we all heard."
"The President of the United States, in the year of our lord 2020, refuses to condemn White supremacists," she said.
Harris added: "People talk about is he dog-whistling -- dog-whistling through a bull horn is what he's doing."
In reality, White supremacists will remain the most "persistent and lethal threat" in the United States through 2021, according to Department of Homeland Security draft documents.
The most recent draft report predicts an "elevated threat environment at least through" early next year, concluding that some US-based violent extremists have capitalized on increased social and political tensions in 2020.