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Biden's visit to the battleground state came a day after Trump, a Republican, made his own trip to Florida - his first campaign trip outside Washington since his Covid-19 diagnosis.
Speaking to about 50 people at a community centre in Broward County in South Florida, Biden said Trump had recklessly dismissed the threat that the virus had posed to their at-risk population.
"To Donald Trump, you’re expendable. You’re forgettable. You’re virtually nobody. That’s how he sees seniors. That’s how he sees you," Biden said.
Trump has touted his management of the coronavirus pandemic.
A Biden win over Trump in Florida would seriously jeopardise the president's chances for re-election, and most recent opinion polls show the Democrat ahead with key demographic groups in the state, particularly seniors. Trump won Florida in 2016 by 1.2 percentage points.
Early voting ahead of Election Day on November 3 is breaking records across the United States, with more than 11.8 million ballots cast so far, including more than 1.6 million in Florida, according to the Elections Project at the University of Florida.
Biden (77) has accused 74-year-old Trump of wanting to gut the Social Security benefit programme for retirees - an allegation that Trump's campaign denied again on Tuesday, saying Biden was trying "to scare seniors for political reasons."
Biden later staged an outdoor, drive-in event in nearby Miramar, Florida, with dozens of cars attending, honking their horns along with his speech.
Trump will travel later on Tuesday to Johnstown, Pennsylvania, for an airport rally in a state that he carried in 2016 by less than 1 percentage point, helping propel him to the White House.
A Reuters/Ipsos poll released on Monday showed Biden with a 7-percentage-point lead over Trump in Pennsylvania, with a majority of voters saying Biden would do a better job of handling the pandemic.
After Tuesday, Trump plans rallies in Iowa, North Carolina, Georgia and Florida later this week as he barnstorms through crucial states just two weeks ahead of the November election.
The president's schedule, however, sends him to areas friendly to him, suggesting that his campaign is concerned with mobilising his conservative base rather than reaching out to undecided voters, many of who live in the nation's suburbs.
FOCUS TURNS TO PANDEMIC
The president returned to the campaign trail on Monday night in Florida for the first time since disclosing he had the coronavirus, throwing out protective masks to supporters but not wearing one himself as he talked about his recovery.
"I went through it now. They say I'm immune. I feel so powerful," Trump told the crowd, who stood shoulder to shoulder, with most not wearing face coverings. "I will kiss everyone in that audience, I will kiss the guys and the beautiful women, I will give you a big fat kiss."
The rally came hours after the White House said Trump had tested negative for Covid-19 on consecutive days and was not infectious to others.
They were the president's first negative Covid-19 tests announced by the White House since Trump said on October 2 that he had contracted the virus. In a memo, White House physician Dr. Sean Conley did not say when the tests were conducted.
Biden has been critical of Trump's management of the pandemic. Trump has worked for months to shift public attention away from the coronavirus, which has infected more than 7.8 million people in the United States, killed more than 214,000 and put millions out of work.
But Trump's illness has put the focus of the campaign's closing stretch squarely on his response to the coronavirus, and opinion polls show Trump losing more ground nationally to Biden as the two White House contenders push toward November.
At the Florida event, Biden said he was disappointed that Trump's bout with the virus had not left him more chastened about his approach to the pandemic.
"The longer Donald Trump is president, the more reckless he seems to get," Biden said. "Thank God we only have three weeks left to go."