Early voting by mail began in Florida weeks ago, with over a million people having already cast their votes.
Mrs Clinton holds a narrow three-point lead over Mr Trump in Florida, according to a new CBS/YouGov poll.
The former secretary of state had 46% of the vote compared with Mr Trump’s 43%, the poll found.
Recent polls have put Mrs Clinton well ahead of her Republican rival both nationally and in several battleground states.
A new CNN/ORC national poll put Mrs Clinton six points ahead among likely voters, at 51% to 45% in a head-to-head with Mr Trump.
Polling in Republican strongholds including Arizona, Georgia and Utah have also shown closer-than-expected races.
But speaking at a rally in St Augustine on Monday, Mr Trump, said: “Folks, we’re winning. We’re winning. We’re winning.”
He once again blamed the media, which he described as being composed of “thieves and crooks” and “almost as crooked as Hillary”, and said the system is “corrupt and rigged and broken”.