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.