Finding out who won the presidency may take days.

Election officials in key swing states are urging Americans to be patient as they tally results, a process that varies from state to state because each has different vote-counting laws. Polls show an extremely tight presidential race, and the closer the election is, the longer it will take to know the outcome.

In the hours and days after polls close, states begin to publish unofficial results that news outlets use to project winners. The states themselves don’t declare winners until weeks later, once the vote tallies have been canvassed and certified.

In 2020, the Associated Press and other news outlets determined Joe Biden had won the presidency four days after the election. The swing states that took the longest then could again be slow this year: Pennsylvania, Arizona and Nevada. If the race is close, other states could join the list.

In addition to legal restrictions on when officials can count mail ballots, long lines at polling places at the end of the day can also slow counting, as voters who are in line at closing time are still allowed to vote.

And then there are ballots from overseas voters and those in the military, which sometimes arrive after Election Day, along with provisional ballots cast by voters whose eligibility is in question. In very close elections, the outcome is unclear until these last ballots are counted.

Here’s a look at the seven battleground states and when results can be expected.

Pennsylvania

In 2020, the Associated Press called the state for Biden on the Saturday after Election Day. Election officials expect the tally to take several days again this year.

Unlike many states, officials there are not allowed to begin processing absentee ballots until 7 a.m. on Election Day. The process is laborious. Workers must check that voters have provided all necessary information, then remove the ballots from the envelopes, unfold them, ensure they haven’t been damaged and feed them into voting tabulators.

Pennsylvania has arcane rules that can lead to challenges over ballots. Absentee ballots must be placed in a secrecy envelope that is then placed in a mailing envelope. The envelopes must be properly dated. If there are mistakes, the ballots may not be counted.

Arizona

Four years ago, Fox News and the Associated Press called Arizona for Biden on election night in a move that others called premature. Most networks waited days to determine if Biden won the state. This time, unofficial results could again take that long.

Arizona allows election officials to process early ballots as soon as they receive them, but many voters wait until Election Day to drop them off.

There are worries that it could take even longer to count votes in Arizona this year, as the ballot is unusually long. In Maricopa County, the state’s most populous county, the races and ballot questions stretch across two pages instead of one — which means voters and officials will have to feed twice as many pieces of paper into vote-counting machines this year. That increases the chances for paper jams that slow down vote counting.

Nevada

In 2020, the Associated Press called Nevada for Biden the Saturday after Election Day.

Voting in Nevada is conducted largely by mail, and Secretary of State Cisco Aguilar (D) has advised counties to start tabulating those ballots as soon as the polls open on Election Day instead of after they close, as they have in the past.

On Election Day, after the last voter has voted, the secretary of state will give counties the green light to release totals for all in-person ballots as well as all mail ballots that arrive before that day. What’s left will be in-person Election Day ballots as well as mail votes arriving that day or later. Election Day precinct totals will start trickling out Tuesday night but are unlikely to be fully reported until later in the week.

Most states require ballots to be returned by Election Day, but Nevada counts mail ballots without a postmark that arrive up to three days after Election Day and those with a postmark up to four days after Election Day. That creates uncertainty on when the results will be known, especially in a close election. The second biggest day for receiving mail ballots in Nevada is typically the day after the election.

Georgia

Networks called Georgia for Biden the Friday after the election in 2020. Election officials expect counting this year to go faster.

Election officials can process absentee ballots as they arrive and can start counting them at 7 a.m. on Election Day.

One hour after the polls close on Election Day — which is scheduled for 7 p.m. Eastern time — state election officials plan to release the counts of all processed mail ballots and early ballots. Early voting has been wildly popular this year, and more than half of registered voters have already cast their ballots. Officials estimate that most county Election Day tallies will be published before midnight Tuesday. They will also publish the total number of ballots cast that day, which will let the public know how many ballots remain to be counted.

A few stragglers will take longer, likely including Fulton County, the state’s largest jurisdiction, which is managing a new counting facility this year.

Michigan

Michigan was called for Biden the day after Election Day. This year, Secretary of State Jocelyn Benson (D) said she expects to have unofficial results for the state by the end of the day Wednesday at the latest.

Large and medium-sized communities this year started to process their mail ballots last week; communities with fewer than 5,000 residents can process them Monday. All jurisdictions will tally the votes on Election Day.

In 2022, Michigan approved a measure allowing in-person early voting. Those who vote that way feed their ballots into tabulators, just as they would on Election Day. That means less work for clerks on Election Day and should help speed up the count.

Wisconsin

In 2020, the count in Wisconsin was completed early in the morning the day after Election Day, and the Associated Press called the state for Biden that afternoon.

Election officials cannot count mail and in-person early ballots until Election Day, which can lead to counting delays.

Milwaukee, a Democratic bastion and the largest city in the state, is expected to be one of the last cities in the state to report its full results because it needs time to tally its early and mail ballots at its central count facility. Milwaukee’s election director expects to have complete unofficial results sometime after midnight. A few other jurisdictions in the state could report their results after that.

North Carolina

The state in the past has reported most results relatively quickly and could provide early clues to how Trump and Harris will fare nationally.

The counting process moves faster in North Carolina, as election officials may remove mail ballots from their envelopes and feed them into tabulators weeks before Election Day. They tally those results on Election Day but cannot announce the results until after the polls close at 7:30 p.m. Eastern time.

Election officials will first report totals for mail ballots and later report totals from those who voted early in person or on Election Day.

Because of damage from Hurricane Helene, some precincts will have to call their results into election offices instead of delivering computer memory sticks to them. This could slow the reporting of results in some counties.

Yvonne Wingett Sanchez and Colby Itkowitz contributed to this report.