A historic election that saw an incumbent president drop his campaign, a woman rise to the top of the Democratic ticket and multiple assassination attempts against the Republican candidate will come to an end on Nov. 5.

But the outcome may not be known on election night.

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

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

An especially tight race, as expected this year, can make it even more difficult to call a winner in the hours after polls close, experts told ABC News. Polls show Vice President Kamala Harris and former President Donald Trump neck-and-neck heading into Election Day.

SEE ALSO: What you need to know about Election Day exit polls

Each state has its own rules to administer elections, including different ways to process mail ballots and deadlines for curing signatures or other issues, which means some may take longer than others to tabulate results.

538 has compiled a complete guide to poll closing times, vote counting and when to expect results in every state.

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

Trump, in 2020, prematurely declared victory before all votes were counted. Misinformation spread online about the integrity of the election as the country awaited a final result and Trump or his allies later challenged the outcome by baselessly claiming widespread fraud, particularly with mail ballots.

“Not knowing the result on election night is not an indication of election malfeasance ever,” Brown emphasized.

SEE ALSO: How voting machines work and why they’re hard to hack | Fact Check

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

In two of those states — Pennsylvania and Wisconsin — mail or absentee ballots cannot even begin to 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 wasn’t 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 or absentee ballots may 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 votes heavily by mail, mail or absentee ballots received before Election Day can be processed and counted upon receipt. But a sizable portion of those ballots get placed in drop boxes on Election Day, and those results may not be collected or counted until polls close, which may hold up 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 help speed up vote counting — including allowing mail or absentee ballots to start being counted 15 days before Election Day.

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

While news organizations often call a winner based on analysis of the vote count as its reported, 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 litigation related to certification, could arise between a race being called by media networks and the results being certified.

On Dec. 17, electors will meet in the states to vote for president and vice president.

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

“We will always prioritize accuracy and security over efficiency,” Michigan’s Democratic Secretary of State Jocelyn Benson recently said on CBS, estimating her state will be able to have a result by end of day on Nov. 6. “Understanding how much people will want those results, we’re still going to make sure the process is secure and accurate before we put anything out to the public.”

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