Biden responded to reports that Russia and Iran obtained voter information as a part of an election disinformation campaign by saying that any country that interferes in American elections will "pay a price."

" They will pay a price. And it's been overwhelmingly clear, this election -- I won't even get into the last one -- this election, that Russia's been involved. China's been involved to some degree," he said.

"Now we learn that -- that Iran is involved. They will pay a price if I'm elected. They're interfering with American sovereignty," Biden added.

In a disturbing allegation, the FBI director and director of national intelligence announced Wednesday night that Russia and Iran have conducted counterintelligence operations secretly to obtain voter registration information about Americans, allowing them to directly reach out to voters in Florida and Alaska.

The director of national intelligence portrayed Trump as the victim of the attack, and did not mention Democratic voters that were targeted.
Biden went on to accuse Trump of being a "Russian pawn" and not speaking out for the country's actions.

"I don't understand why this president is unwilling to take on Putin when he's actually paying bounties to kill American soldiers in Afghanistan, when he's engaged in activities that are trying to destabilize all of NATO. I don't know why he doesn't do it, but it's worth asking the question," he said.

TRUMP'S CLAIM: "Frankly, (Biden) ran the H1N1, swine flu, and it was a total disaster, far less lethal, but it was a total disaster. Had that had this kind of numbers, 700,000 people would be dead right now."

FACT CHECK: While Trump is correct that the H1N1 virus was much less lethal than COVID-19, it is misleading to call the Obama administration's response a "failure."

The Centers for Disease Control and Prevention estimates up to 575,000 lives were lost to the swine flu worldwide. Of those, fewer than 13,000 were American, due in part to the Obama administration's "complex, multi-faceted and long-term response," the CDC later wrote. Thus far, COVID-19 has taken the lives of over 210,000 Americans, a little over eight months since the first known case of the virus was discovered in the United States.

"The team, in my opinion, in 2009, really demonstrated that the planning was worth it. Nothing is ever perfect. But I felt just so impressed and so proud of the job CDC did in 2009," Dr. Julie Gerberding, a CDC director during the George W. Bush administration, told ABC News.

-ABC News' John Verhovek

Before moving on to election security, Welker asked Trump about his recent public criticisms of Dr. Anthony Fauci.

"President Trump, this week you called Dr. Anthony Fauci, the nation's best known infectious disease expert, quote, 'a disaster.' You described him and other medical experts as, quote, 'idiots'. If you're not listening to them, who are you listening to?" she asked.

"I'm listening to all of them, including Anthony," Trump said. "I get along very well with Anthony, but he did say don't wear masks. He did say, as you know, this is not going to be a problem. I think he's a Democrat, but that's okay."

Fauci is registered as an Independent and said not to wear a mask at the start of the pandemic as the task force was concerned with having enough personal protective equipment for health care workers. Once the science became clearer, he recommended the practice.

Biden responded by saying it was Trump who panicked when the virus hit U.S. soil, not the American people, drawing on interviews with journalist Bob Woodward.

"Think about what the president knew in January and didn't tell the American people. He was told this was a serious virus that spread in the air and it was much worse -- much worse than the flu," Biden said. "Americans don't panic. He panicked."

Biden responded to concerns that another shutdown could harm the economy, already hurting from closures caused by the pandemic.

"I'm going to shut down the virus, not the country," he said. "It's his ineptitude that caused the virus -- caused the country to have to shut down in large part. Why businesses have gone under, why schools are closed, why so many people have lost their living and why they're concerned."

After attacking Trump for golfing during the pandemic, Biden said he hadn't ruled out additional shutdowns if necessary.

"You need standards," he said. "The standard is if you have a reproduction rate in a community that's above a certain level, everybody says slow up. More social distancing. Do not open bars and do not open gymnasiums."

In rebuttal, Trump criticized Democratic governors in several states for shutting down in response to the pandemic.

"They're shut down so tight and they're dying," he said. "They're dying. And he supports all these people. All he talks about is shutdowns. No, we're not going to shut down. And we have to open our schools."

In an example of how young people have better outcomes with coronavirus, the president mentioned his son Baron who tested positive.

"By the time I spoke to the doctor the second time, he was fine. It just went away. Young people -- I guess it's their immune system," he said.

Biden said if he is elected he would pass Obamacare with a public option, deeming it "Bidencare," and outlining his plan.

"If you qualify for Medicaid and you do not have the wherewithal in your state to get Medicaid, you're automatically enrolled, providing competition for insurance companies," Biden said.

"Secondly, we're going to make sure we reduce the premiums and reduce drug prices by making sure that there's competition that doesn't exist now, by allowing Medicare to negotiate drug prices with the insurance companies," he said.

Biden said private insurance would be protected under his plan.

"Thirdly, the idea that I want to eliminate private insurance -- the reason why I had such a fight for -- with 20 candidates for the nomination, was I support private insurance," Biden said. "Not one single person with private insurance would lose their insurance under my plan, nor did they under Obamacare."

"Lastly, we're going to make sure we're in a situation that we actually protect pre-existing conditions. There's no way he can protect pre-existing conditions. None. Zero," Biden said, as the Trump administration seeks to dismantle the entire Affordable Care Act which currently protects them.

Trump said earlier, "I'd like to terminate Obamacare, come up with a brand-new beautiful health care."