But there's another reason results are likely to be later than usual this year: Some states lag in how they handle mailed ballots before the tabulating ever starts.
A mail-in ballot cannot be counted until election officials verify that it was returned by a registered voter. The ballot is in an unmarked envelope, which is mailed inside a larger outer envelope with a place for the voter's name and signature. The name and often the signature must be checked against a voter registration database to verify the ballot's authenticity.
If the ballot isn't signed or the signature doesn't match what's on file, the voter can be contacted to resolve the discrepancy. All of that takes time. Once verified, the ballot itself, inside the unmarked envelope, is set aside until the counting begins.
A timeline from the federal Election Assistance Commission, which was set up after the chaotic 2000 presidential election, notes that states with long experience in handling large volumes of mailed ballots begin to verify them about 20 days before Election Day. In 35 states, the process starts early, and in 12 of those states, election officials can begin checking the validity of mailed ballots as soon as they're received.
"If your goal is to know as much as possible on election night, being able to process them in advance is absolutely going to be important," said the agency's commissioner, Ben Hovland.
Kentucky's secretary of state, Michael Adams, told the House Homeland Security Committee last week that for the first time, election officials were allowed during the June primary to start the processing early.
"It took a good week for us to get all the ballots counted, but it would have been even longer without that," he said. The state plans to do the same for the general election, starting the process Sept. 21.
But in 11 other states, including the presidential battlegrounds of Michigan and Pennsylvania, election officials can't even start the process until Election Day. And in three other states, they can't begin until the polls close.
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During Pennsylvania's primary in June, mail-in ballots were still being counted a week after the election. Pennsylvania's State Department, which administers elections, is urging the state Legislature to act quickly to allow the processing to begin three weeks before this year's general election, a proposal endorsed by Gov. Tom Wolf, a Democrat.
Giving states more time for processing wouldn't solve the problem of ballots that arrive too late, by far the main reason mail-in ballots are rejected. During Michigan's primary this month, for example, about 60 percent of the mail-in ballots that were rejected came in after the legal deadline.
A longer processing window gives voters a chance to fix their mistakes, cutting down on the number of ballots that can't be verified.
"That is, of course, one of the many reasons why all of this should be happening before election night," said Trevor Potter, a former chairman of the Federal Election Commission.
Once mail-in ballots are verified, a second issue comes into play. They often take longer to count, especially when voters fail to follow instructions about how to fill in blanks to indicate their choices. Small bipartisan groups assemble to examine disputed ballots, trying to determine voters' intent.
"There are states where that will be fought ballot by ballot," Potter said.
Rick Pildes, an election law expert at New York University School of Law, said lawsuits are sure to follow unless local canvassing boards resolve the disputes in a similar way.
"There will be litigation if it turns out that states end up treating these ballots differently in different parts of the state, assuming it's consequential enough," he said.
Election administrators and legal experts have a consistent message. They urge voters to cast their ballots in person whenever possible or to request and return their mail-in ballots at the earliest possible date.
CLARIFICATION (Sept. 1, 2020, 3:09 p.m. ET): An earlier version of this article referred imprecisely to the share of mailed ballots that were rejected in Michigan. Roughly 60 percent of the mail-in ballots that were rejected came in after the deadline; the state did not reject 60 percent of all ballots that were mailed in.