While I've been talking a lot about the polls, the difference between voter sentiment (which polling gauges) and valid votes actually cast should be noted. Factors affecting the latter include voter turnout, spoiled ballots, ballot submission deadlines, ballot design, polling place operations, and more. It is possible that while more Americans prefer Biden over Trump, the valid ballots actually cast and counted could produce a Trump win.
A recent example of submitted but uncounted mail-in ballots is provided by Florida's September primary election results. Of all mail-in ballots submitted, 1.5 percent were rejected because of missed deadlines or technical flaws. (
source). This compares to the approximate 1.0 percent rejection rate in the 2016 and 2018 Florida general elections. (
source) Note that general elections include independent voters while primary elections include Democratic and Republican voters only.
"Democrats cast 50 percent of absentee ballots in the primary and accounted for 47 percent of the mail-in votes that didn’t count, ..." [Professor Dan Smith said]. "Republicans, who for years dominated absentee voting and have an electorate familiar with the process, cast 34 percent of absentee ballots in the primary and made up 31 percent of mail-in votes that didn’t count." (
source)
In other words, of all the FL Sept. primary election ballots mailed in, 1.5% were spoiled ballots that were not counted. That came to about 35,000 spoiled ballots. Of those, 47% of them (16,450 ballots) were submitted by registered Democrat voters, and 31% (10,850) were submitted by registered Republicans.
98.5 % of all mail-in ballots were deemed valid and counted. In recent general elections (2018, 2016), about 99% of all mail-in ballots have been deemed valid and counted.
Is the Florida system reliable?
Politico:
"Florida Secretary of State Laurel Lee told local election supervisors this week that she was “very confident in our ability to administer an orderly vote-by-mail process with a high level of integrity.” The state has had vote-by-mail available for decades and no-excuse mail voting for nearly 20 years.
“This is a method of voting we are very familiar with,” Lee said.
