Mail-in and absentee ballots, which voters are using at historic levels, will slow down tallying on Tuesday and will likely delay the results in some of the battleground states critical to determining whether Vice President Kamala Harris or former President Donald Trump wins the election.

Election officials in some of the closest swing states have warned the public to be patient as they await results that may not be delivered on Election Night, especially if the outcome is as close as current polling suggests. 

Arizona

If the winner of the presidential election winner comes down to Arizona, and the margin between Ms. Harris and Mr. Trump is extremely narrow, prepare to wait days for the results.

State officials said it could take up to 12 days to count all of the state’s ballots, but the results are expected much sooner — unless the margin is narrow and the late-tallied votes are needed to determine a winner. Expect to wait longer if one of the candidates wins by less than 0.5%, which, under a 2022 law, will trigger an automatic recount.

President Biden won Arizona by 0.3% in 2020 when the law required a recount on margins of 0.1% or narrower. 

In Maricopa County, home to more than 60% of the state’s population, election officials say they anticipate up to half a million voters will drop off absentee ballots on Election Day. This could delay results as workers tally the results of both in-person voting and those last-minute ballots. 

This year’s ballot in Maricopa County is a long one. Voters must make choices that extend onto two pages. 

Nevada

Like Arizona, a very close result will likely delay projecting a winner because every vote will have to be tallied in the Silver State, where counting every ballot traditionally takes a few days.

Nevada’s polls close at 10 p.m. ET, the latest of all the battleground states, and unlike other battleground states, mail-in ballots postmarked by Election Day can be counted if they are received up to four days later, on Nov. 9. 

Nevada mailed ballots to every voter in the state. 

The Associated Press called the 2020 presidential election in Nevada for Mr. Biden four days after the election. At that time, 88% of the votes were counted. Mr. Biden ultimately won the state by 2.3%.

Pennsylvania

Election experts predict absentee ballot processing rules will slow down the results in Pennsylvania. The state does not allow election workers to begin processing ballots until Election Day.

Polls show Pennsylvania is virtually a tie between Ms. Harris and Mr. Trump. A close result, combined with expanded voting by mail started during the COVID-19 pandemic, means the delayed processing will almost guarantee a delayed result.

“It will likely take days to count all of the absentee ballots necessary to determine the winner in a state with such close margins,” Matt Germer and Chris Mcisaac predicted for the policy research firm R Street. 

Mr. Biden beat Mr. Trump in Pennsylvania by about 1% of the vote in 2020. AP projected the Keystone State for Mr. Biden four days after the Nov. 3 election, but the counting wasn’t finished. The state did not certify the results until Nov. 24, and even then, two counties had not certified their results. 

Secretary of the Commonwealth Al Schmidt told CBS News in October: “The message is, please be patient.”

Wisconsin

Like Pennsylvania, the Badger State is prohibited from counting absentee ballots until Election Day, which could slow down determining a winner. Results may also be slowed in Milwaukee and dozens of other cities that require absentee ballots to be counted at a single location, R Street reported. Mr. Biden won Wisconsin by 0.63% in 2020.

Michigan

The Wolverine State changed the law to allow election workers to process ballots before election day, but don’t expect to learn who won there on Tuesday. Plan to wait at least an extra day or so, Michigan Secretary of State Jocelyn Benson said. Newly permitted early in-person voting could also require more time for tallying. 

“I would estimate by the end of the day on Wednesday as the best guess on how we’ll perform,” Ms. Benson told CBS News. Mr. Biden won Michigan by less than 3% of the vote in 2020.

Georgia

Secretary of State Brad Raffensperger said he expects 75% of Georgia’s vote total to be reported by 8 p.m. on Election Night, which bodes well for the full results to come in later in the evening, but it could take days to learn the winner if results are as close as polls suggest. 

A state judge struck down a rule requiring ballots to be counted by hand, which should ensure fewer delays. But Mr. Raffensperger said if the margin is very narrow, the final result may not be known until the overseas ballots are counted, which he said would happen “no later than Friday.”

North Carolina

The 2020 election wasn’t called for former President Trump until 10 days after the polls closed. The result could come faster this year thanks to a new state law that requires all ballots to be received by Election Day. The same law also banned counties from processing ballots cast during the early voting period until 7:30 p.m. on Election Night, which officials said could delay results by an hour or more. 

North Carolina voters will also have to provide a photo ID at the polls for the first time.

“This requirement may lead to more voters casting provisional ballots, which take longer to count and could lead to a longer wait before complete results are determined,” Derek Tisler, counsel for the Brennan Center’s Elections and Government Program, reported.

• Susan Ferrechio can be reached at sferrechio@washingtontimes.com.