A historic election in which a sitting president abandoned his campaign, a woman rose to the top of the Democratic party and multiple assassination attempts against the Republican candidate will come to an end on November 5.

But the outcome may not be known on election night.

It took four days for the 2020 race to be called for President Joe Biden as an expansion of mail-in voting and other changes made to help Americans participate during a global pandemic delayed the count in several key states.

“It could take a few days and sometimes longer,” said Barry Burden, director of the University of Wisconsin-Madison Elections Research Center.

A particularly tight race, like the one expected this year, could make it even more difficult to declare a winner in the hours after polls close, experts told ABC News. Polls show Vice President Kamala Harris and former President Donald Trump are neck-and-neck heading into Election Day.

Each state has its own rules for administering elections, including different ways to process mail-in ballots and deadlines for signature recovery or other issues, meaning some states may take longer than others to tabulate results to bring.

538 has put together a complete guide to poll closing times, vote counting, and when to expect results in each state.

“There are several things that need to be done because there are safeguards in place to minimize the possibility of fraud,” said Mitchell Brown, a professor of political science at Auburn University. “And so in states that have these rules, it takes a while for all the ballots to be processed.”

Trump prematurely declared victory in 2020 before all votes had been counted. Disinformation about the integrity of the election spread online as the country waited for a final result and Trump or his allies later cast doubt on the outcome by baselessly claiming widespread fraud, especially with mail-in ballots.

“Not knowing the results on election night is not an indication that election crimes were ever committed,” Brown emphasized.

All eyes will be on the seven swing states that will likely determine whether Harris or Trump will win the Electoral College: Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin.

In two of those states – Pennsylvania and Wisconsin – mail-in or absentee ballots cannot even be processed until the morning of Election Day. That includes opening envelopes, verifying voter information and preparing them to be scanned before they can be counted, which can lead to delays.

In 2020, Wisconsin was not called for either candidate until the day after Election Day, and Pennsylvania was called the Saturday after Election Day.

In other key battleground states, mail-in or absentee ballots can be processed but cannot be counted until Election Day, according to the National Conference of State Legislatures. That includes Georgia, Michigan and North Carolina.

In Arizona, a state that has high rates of voting by mail, mail-in or absentee ballots received before Election Day can be processed and counted upon receipt. But a significant portion of those ballots are placed in drop boxes on Election Day, and those results may not be collected or counted until the polls close, which could yield a clear result depending on how close the race is.

In Nevada, another state where the presidential race wasn’t called until the Saturday after Election Day in 2020, some changes were made to speed up vote counting, including allowing mail-in or absentee ballots to be mailed 15 days before the election day are counted.

“It’s really a product of the laws and depending on where the Electoral College spotlight is in a given year, it could mean a faster or slower count,” Burden said.

Although news organizations often declare a winner based on the analysis of vote counts as reported, the results are not official until states certify them. States have their own certification deadlines, some of which extend into December, according to the Election Assistance Commission.

Recounts and legal challenges, especially lawsuits related to certification, could arise between a race being called by media networks and the results being certified.

On December 17, voters in the United States will meet to vote for a president and vice president.

Election officials in some key states are already warning that results may not come in on election night, and that this is normal.

“We will always prioritize accuracy and safety over efficiency,” Democratic Michigan Secretary of State Jocelyn Benson recently told CBS, estimating her state could have a result by the end of the day on Nov. 6 . “Understanding how much people want With these results, we are still going to ensure that the process is safe and accurate before we release anything to the public.”

“We want to make sure that we have an accurate count, and just like we did in 2020, we have free and fair, secure elections,” Pennsylvania Gov. Josh Shapiro said on ABC’s “This Week.”