Unless polling that portends a paper-thin margin between Vice President Kamala Harris and former President Trump is completely wrong — a possibility because of dizzying changes in how voters cast ballots as well as a pandemic-prompted overhaul of election laws — Americans should plan on going to bed Tuesday night without knowing who won the White House.

Election experts argue that a delay in knowing the result, in part because of the patchwork of rules that dictate how votes are counted, notably in battleground states, is proof of ballot-counters’ vigilance about accurately tabulating the vote. But they also worry that any delay will feed the growing skepticism many Americans have about the sanctity of the nation’s electoral process.

“People are accustomed to turning on their TV late election night and seeing a winner splashed across the screen,” said Jessica Levinson, an election law professor at Loyola Law School. “I think that’s largely a thing of the past. It’s hard to think about how that could happen in a race this close.”

She pointed to changing behavior among voters about how they cast their ballots, as well as varying rules about when mail ballots can be tallied as well as how late mail ballots that are postmarked on election day will be accepted. But she also worries that in today’s polarized climate, election officials doing their due diligence to properly count the vote will be misinterpreted as opportunities for fraud.

“Everybody knows what a photo finish is,” Levinson said. Expecting a clear result on election night “is kind of like declaring victory before you can develop the film. Or it’s like declaring victory before you do the last lap, because honestly the last lap is the counting.”

Charles Stewart III, the director of the MIT Election Data and Science Lab, said ultimately it will come down to the margin of victory in the race.

“In all of these states, part of the answer really depends on how close the election actually is,” he said. “I say that because it really only matters if the election really is close.”

More than 76 million Americans have voted as of Sunday, either through mail ballots or in-person early voting, according to a tracker by the University of Florida’s Election Lab. Several states have reported record-breaking early returns, including two that could tip the presidential race — Georgia and North Carolina.

More than 4 million Georgians had cast ballots as of Friday, according to the secretary of state’s office. The number far eclipses prior elections.

“Georgia voters know we’ve made it easy to cast a ballot. It’s really that simple,” Secretary of State Brad Raffensperger, who famously fought Trump’s effort to increase his 2020 vote count in Georgia, said in a statement.

Some states changed voting rules because of the pandemic, including California, which now mails ballots to every registered voter, or in the aftermath of the contentious 2020 election. That includes a number of the battleground states that will determine who is elected president. Additionally some of these states have restrictions on when mail ballots can be received by or tallied.

In 2020, then-President Trump spoke out against mail ballots — historically a voting practice favored by Republicans. The end result was voters seeing candidates who appeared to be leading after the polls closed on election day losing once all the votes were counted. This led to widespread and false conspiracy theories in states like Michigan about the undulating vote count.

If Tuesday’s election is similarly close, there will likely be a repeat of this scenario.

“If the race comes down to a couple thousand ballots in a key state, it could be days or weeks before we know the winner of the presidential election,” said Rachel Orey, director of the Bipartisan Policy Center’s Elections Project. “It really comes down to the margin of victory.”

Rick Hasen, a campaign-finance law professor at UCLA whose latest book is “A Real Right to Vote: How a Constitutional Amendment Can Safeguard American Democracy,” said the rules in Pennsylvania, Wisconsin, Nevada and Arizona could hinder the declaration of a winner in a particularly tight election.

“If it comes down to those states, there could be a delay for a few days for different reasons,” Hasen said. “Pennsylvania and Wisconsin don’t allow the processing of absentee ballots before election day. Wisconsin moves faster because it’s smaller, but the bottleneck is Milwaukee.”

“Nevada is a different problem, they allow ballots to arrive four days” if they are postmarked on election day, he said. “And in Arizona, they just have a ton of mail ballots and it takes a very long time, like California.”

Another complication in Arizona is that voters in Maricopa County, the state’s most populous, are being asked to complete the longest ballot they have seen in nearly two decades, Orey said. That could lead to delays at voting centers as voters take longer to cast their ballots, slow down tabulation and potentially create paper jams in counting machines because the ballot is two pages, front and back

However, there are bright spots in other key states, he said. Georgia and North Carolina count votes relatively quickly, and Michigan changed its rules and is likely to be faster than it was four years ago.

A key question is whether the election turns on a small number of votes in one state, such as what occurred during the 2000 race between Al Gore and George W. Bush.

“If it comes down to a few hundred or a few thousand votes in one state, it could be protracted for weeks,” Hasen said, noting the chances of a repeat of that deeply contentious election that was eventually decided by the Supreme Court are small but not impossible. “Then all bets are off … It will be battling ballot-by-ballot, trench warfare.”

Stewart added that both Harris’ and Trump’s campaigns have been preparing for this possibility.

“They are loaded for bear, as they say,” he said. “Hundreds of lawyers and hundreds of briefs are ready to go.”

Hasen’s advice to anxious voters: Recognize once the polls close on Tuesday that delays in learning the results are a sign that the system is working, and to remain calm.

On Tuesday night, “Everyone needs to take a breath, have some patience, have a glass of wine, and get up the next day and do it all over again,” he said. “Maybe at the end of the week, we know what the answer is. Unless it’s a blowout.”