A Winner on Election Day in November? Don’t Count on It

“That suggests something’s wrong,” he said.

“The need to take a longer time to process and count these ballots is a sign of the process working,” Mr. Masterson added. “It is in no way an indication of anything malicious.”

In the presidential race, the vote margins in most places are expected to be wide enough to allow media organizations, such as The Associated Press, to project winners of individual states even before all the votes are tabulated. But delays are widely expected in at least some key states, which could leave the country in momentary political limbo if neither candidate has reached the 270 electoral votes required to win the presidency.

David Scott, deputy managing editor for The A.P., said how long it takes to call the presidential race in November will depend on the closeness of the race and whatever rules states impose between now and Election Day. The A.P. will have to account for the huge wave of mail-in votes, especially after incorrectly declaring two races in Georgia were headed to runoffs this month.

In a sign of its cautious approach, the organization has not yet declared that Representative Eliot L. Engel, Democrat of New York, lost on Tuesday, even as he trails his challenger, Jamaal Bowman, by nearly 27 percentage points.

“I’ve been saying to anyone who will listen that I do not think we will know who won the presidential election on Nov. 3,” said Matthew Weil, director of the elections project at the Bipartisan Policy Center, a think tank.

Mr. Weil pointed to three states in particular he expected to be hot spots for slow counting trouble: Michigan, Pennsylvania and Wisconsin. Those three states, which were key to Mr. Trump’s victory in 2016, are seen by both the Trump and Biden campaigns as linchpins to the path to victory.

And all three of those states have one rule in common: they do not currently allow the tabulating of mail-in ballots until the day of the election, though election officials are pressing to relax those restrictions.