The Vermont senator's remarks came hours after Trump claimed during a Labor Day press conference Monday that "we're going to have a vaccine very soon, maybe even before a very special day".
Trump is also fighting to maintain support among veterans and those serving in the USA military after a magazine, citing four unnamed people, reported that he had referred to Marines buried in an American cemetery near Paris as "losers" and "suckers" and declined to visit their graves during a 2018 trip to France.
Biden's visit to Pennsylvania on Monday kicked off a flurry of travel to battleground states this week by both Biden and Trump as some opinion polls show the race tightening with less than 60 days to go until the November 3 election. "We have to give the resources to the doctors and the scientists to do their work", Sanders said.
Biden's day in a key swing state, nearly "normal" by covid-era standards, stood in stark contrast to Trump, who played golf and held a news conference during which he ranted about and insulted his opponents.
Trump is hardly the first candidate to play hardball during a presidential race.
"He didn't do it before, why would he do it now?"
The Taoiseach said he recently spoke on the phone with Mick Mulvaney, President Trump's special envoy for Northern Ireland. "The United States experienced the smallest contraction of any western nation.You look around and see how we're doing compared to any other nation, our rise is spectacular, and we're rebounding much more quickly from the pandemic".
Biden, who was already in Pennsylvania on Labor Day Monday, will be in MI on Wednesday, while his vice presidential pick Kamala Harris goes to Florida and his wife Jill Biden visits Minnesota - a state Trump is pushing hard to flip to his side on November 3.
Serena Williams battles to win in 3 sets, reach US Open quarterfinals
However, the doubts that have crept into Williams' formidable psyche resurfaced with two errors that gave Sakkari the set. But it was clear to anyone watching that 3-year-old Olympia was very much enjoying watching her mama play.
Still, polls show Biden maintaining a persistent, if not insurmountable, lead over Trump, with both men increasingly focusing on key upper Midwest states like Wisconsin, where polling is closer.
Democrats are hardly counting Trump out.
Mike McCormick, who was a White House stenographer for 15 years, said Biden's decreased sharpness is evident when watching him speak compared to just three years ago. "We will be transparent with the data".
"We will make America into the manufacturing superpower of the world and will end our reliance on China once and for all", he added. She's been sticking to digital fundraisers and organizing events.
President Trump boards Air Force One on Tuesday at Andrews Air Force Base in Maryland.
"If money was the only factor determining winners and losers in politics", he said, "then Jeb Bush would have been the nominee in 2016 and we'd have a second President Clinton".
Prior to Biden's visit, he hadn't campaigned in Wisconsin since October 2018.
He also pointed to the campaign's early expenditures as paying off, particularly on field staff in battleground states and said that Biden could not replicate that in the little time left until Election Day.
Trump described Biden, whom he trails in national polls, as a threat to the economy and "stupid", while Biden took aim at Trump's reported disparaging of fallen troops.