After Vice President Mike Pence said the Democratic ticket would ban fracking, Harris denied it.
"Joe Biden will not ban fracking. That is a fact. That is a fact," she said.
Trump, in response, tweeted a video of Harris saying in September 2019, "There's no question I'm in favor of banning fracking."
Biden's rhetoric on fracking has at times been confusing, but he has said he favors a ban on "new fracking" and won't seek to end existing permits. On his website, Biden calls for "banning new oil and gas permitting on public lands and waters."
It is an issue in the election because thousands of jobs in swing states like Pennsylvania are linked to fracking, or hydraulic fracturing.
Later in the debate, Trump tweeted an ad about American jobs lost to China during Biden's time in public office.
The president also said: "Mike Pence is doing GREAT! She is a gaffe machine." And later on, he tweeted a video featuring his Supreme Court nominee, federal appeals Judge Amy Coney Barrett, who came up in the debate.
Biden responded after Pence accused the Biden-Harris ticket of seeking to raise taxes: "Let me be clear: A Biden-Harris Administration won't increase taxes by a dime on anyone making less than $400,000 a year."
And Biden praised his running mate earlier in the debate, saying she's "smart, she's experienced, she's a proven fighter for the middle class."
"She'll be an incredible Vice President," he said.
Toward the end, Biden alluded to a fly that sat on Pence's head for a period of time, with a request for donations: "Pitch in $5 to help this campaign fly," he tweeted before linking to a donations page.