Mr Biden told supporters: "You hold the power. If Florida goes blue [Democratic], it's over."
Celebrating soaring economic figures, Mr Trump, a Republican, said of his rival: "He's going to lock you down."
With just five days to go until election day, Mr Biden has a solid lead nationally in opinion polls.
But his advantage looks less assured in the battleground states, such as Florida, that will decide who ultimately wins the White House.
More than 81 million people have already voted, 52 million of them by mail, setting the US on course for its highest electoral turnout rate in more than a century.