A historic election that saw a sitting president abandon his campaign, a woman rise to the top of the Democratic ticket and multiple assassination attempts against the Republican candidate will end on November 5th.

But the result will probably not be known on election evening.

President Joe Biden's 2020 race took four days to be called as the expansion of mail-in voting and other changes made to make it easier for Americans to participate amid a global pandemic delayed counting in several key states.

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

A particularly close race, as expected this year, may make it even harder 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 heading into Election Day neck and neck.

Each state has its own rules for conducting elections, including different methods for processing mail-in ballots and deadlines for fixing signatures or other issues, meaning tabulation of results may take longer in some states than others.

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

“There are a number of things that need to be done because there are safeguards in place to minimize the possibility of fraud,” said Mitchell Brown, a political science professor 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 the votes were counted. Misinformation about the integrity of the election spread online as the country waited for the final result, with Trump or his allies later challenging the result with baseless claims of widespread fraud, particularly in mail-in ballots.

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

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

In two of those states – Pennsylvania and Wisconsin – absentee or absentee ballots cannot be processed until the morning of Election Day. This includes opening the envelopes, checking voter information and preparing them for scanning before they can be counted, which can cause delays.

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

According to the National Conference of State Legislatures, absentee or mail-in ballots in other key battleground states may be able to be processed but not counted until Election Day. These include Georgia, Michigan and North Carolina.

In Arizona, a state that predominantly votes by absentee, absentee or mail-in ballots, ballots received before Election Day can be processed and counted upon receipt. However, a significant portion of these ballots are dropped into mailboxes on Election Day, and results may not be collected or counted until after polls close, which could produce a clear result depending on the tightness of the campaign.

In Nevada, another state where the presidential race wasn't played until the Saturday after Election Day in 2020, some changes were made to speed up the counting of votes – including allowing absentee or absentee ballots to be counted can begin as early as 15 days before election day.

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

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

Between the time a race is called by media networks and the certification of the results, recounts and legal challenges, particularly litigation related to certification, can occur.

On December 17, state voters will meet to vote for president and vice president.

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

“For us, accuracy and security will always trump efficiency,” Michigan Secretary of State Jocelyn Benson, a Democrat, said recently on CBS, estimating her state will be able to produce a result by the end of the day on Nov. 6. “We understand how much people want. “After these results, we will still ensure that the process is safe and correct before releasing anything to the public.”

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