He told them that the president saw seniors, who have been more at risk in the pandemic, as "expendable".
There are sharp policy differences between the two candidates on Covid-19.
In Pennsylvania, Mr Trump told thousands of supporters he felt like "Superman" after his Covid treatment.
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Opinion polls suggest Mr Biden has a 10-point advantage over Mr Trump nationally, but his lead in some key states is narrower. In Florida, the Democrat is 3.7 percentage points ahead, according to an average of polls collated by Real Clear Politics.
Mr Trump narrowly won Florida in 2016 in a result buoyed by senior voters. But the latest polls suggest a shift away from the Republican among them this time around.