After the Biden campaign had announced no public events for the day, he visited with supporters at a voter field office and activation center in Chester, Pennsylvania, in Delaware County, while Trump also rallied in the battleground state, and delivered impromptu remarks attacking the president's response to the pandemic.

"What in the hell is the matter with this man?" Biden said, arguing the Trump administration is "giving up" on its response as Trump insists cases are "leveling out" when they’re actually on the rise.

"I'm not going to give up. I don't know what we'll inherit on January 21st, but at the rate he's going it's not going to be good," Biden continued.

Asked about the outdoor event the White House is planning tonight to celebrate the anticipated Senate confirmation of Judge Amy Coney Barrett to the Supreme Court, Biden said it sets a bad example to all Americans when the president holds "superspreader events."

"I just hope he is willing to have learned a lesson and there will be significant social distancing. I don't blame him for celebrating. There's a lot of things we could be doing having massive crowds, but the fact is that it's just not appropriate now," Biden said. "It shouldn't be a huge crowd, whether it's outside or not."

Defending himself against criticism from Trump that he’s in "hiding" in the final days of campaigning, Biden noted he’ll be visiting Iowa, Wisconsin, Florida and Georgia this week but also took the chance to differentiate his campaign events from Trump’s.

“We're going to be traveling -- continue to travel, but the big difference between us -- and the reason why it looks like we're not traveling, we're not putting on superspreaders,” Biden said. “We are doing what we're doing here. Everybody's wearing a mask and trying as best we can to be socially distanced and that’s what we do."

At the second of three events in Pennsylvania Monday, Trump told supporters at a rally in Lititz while he wants any transition to be done "absolutely by the book" but that it’s "very hypocritical" for Democrats and other critics to expect a friendly transfer of power.

"We want it to be done absolutely by the book, and we want to go absolutely by our great Constitution. But you know what, when they start talking about this friendly wonderful transition. And then you see what they put us through. It's really very hypocritical isn’t it?" Trump told the fired up crowd. "On November 3rd we must finish the job and drain that very deep and very nasty swamp once."

Ticking through the battleground state Monday, Trump urged voters to turnout in stronger numbers than in 2016 as he narrowly won the state then by only .72%.

"For the last four years, you've seen me fight for you and now I am relying on you to deliver a historic victory for our country. Bigger even than four years ago," Trump said.

Trump continued to brush off the effects of COVID-19 as cases and hospitalizations rise across the country, saying, "One day you get it, and, that's OK. You get better. We have such great, I think, cures." He again raised the diagnosis of his youngest son, Barron, 14, to downplay the effects of the disease on children. According to the latest report from the American Academy of Pediatrics, at least 120 children have died from COVID-19 since late March.

"I mean, I got it and I'm here. We have great medicines and therapeutics," he said. “You know who else got it? Barron. My very tall son… It’s gone. Young kids, they have a strong immune system. I've gained such respect for them.”

“It's absolutely an amazing thing what we've done in a period of seven months because, you know, I'm the president of the United States,” Trump added.

Some Amish supporters, without masks, stood directly behind Trump at the rally, and the demographic received a nod from Trump.

“We have Pennsylvania Dutch. Don't tell anyone. You know, they -- they’re great people. They're great people, but they're not known for going out and voting for a lot of reasons. Don't tell anybody, but the Pennsylvania Dutch are voting en masse,” Trump said.

In slamming trade positions taken by the Obama administration, Trump also said he wants the U.S. to be a “developing nation” as the World Trade Organization still recognizes China as a developing country.

“They are considered a developing nation and I said so we are a developing nation, too. We’re developing, they are developing, we are developing, they get big advantages,” Trump said. “We want to be a developing nation also.”

Democratic vice-presidential nominee Kamala Harris, the first woman of color on the ticket of major party, responded to attacks and slights she's endured from Trump, including mispronouncing her name and referring to her as a "female socialist," during an interview with ABC's "The View."

"It's so predictable coming from him. I mean it's childish, it's name-calling on behalf of the president of the United States, and, again, the American people deserve so much more from their president," said Harris. "You know, look, the name-calling is not new to me -- it's not new to anybody who played on the playground as a child. But this is not the playground."

She also took the opportunity to ease concerns around Biden's stance on fossil fuels after Biden said at the last debate he would transition from the oil industry overtime and end fossil fuel subsidies, and the Trump campaign seized on the comments.

"Without any ambiguity, Joe is clear. We will not ban fracking. And let's clear up further things and we will not raise taxes on anyone making less than $400,000 a year, period, full stop," said Harris. “Joe is also committed to understanding that we have to also acknowledge the science in a way that frankly the current president does not, on the COVID issue and on climate."

Harris will travel to Nevada on Tuesday, and according to a source familiar with plans, she will be in Texas on Friday -- the first Democratic vice presidential candidate to campaign in the state in decades -- as Democrats eye the GOP stronghold. Texas currently leads the nation with the largest number of early votes.

-ABC News’ Averi Harper

Trump's first rally of the day was in Allentown, in Pennsylvania's Lehigh County and the larger Lehigh Valley region in the northeastern part of the state.

Hillary Clinton won 50% of the county in 2016, with roughly 7,600 more votes than Trump. But he won nearby Northampton County by 5,400 votes, on the way to winning Pennsylvania by roughly 44,000 votes. Given that Northampton County is one of the few pivot counties that went for President Obama in 2008 and 2012 before turning to Trump, the president's visit represents an effort to keep his supporters in the suburbs in play.

The area was represented by U.S. Rep. Pat Meehan, a Republican, from 2011-2018, until he resigned amid a sexual harassment scandal. After redistricting, the area is now represented by Rep. Susan Wild, a Democrat.

His second stop brought him to Lititz in Lancaster County where he easily defeated Clinton by roughly 50,000 votes four years ago. He also won with roughly the same margin as Mitt Romney did in 2012, though more people voted in the 2016 race for both candidates than four years prior.

Pence has also visited the area twice since August.

Trump holds a third rally in Martinsburg, in Blair County, where he won roughly 75% of the vote in 2016. The county in central Pennsylvania between Harrisburg and Pittsburgh is a reliably conservative region of the state -- where Trump needs to run up his numbers to help keep Pennsylvania in GOP hands in November.

-ABC News' Ben Siegel