In a nail-biter election Nov. 5, expect the losing side to challenge the results in court.

After former President Trump’s 2020 defeat, his supporters filed numerous of lawsuits nationwide raising questions about ballots, counting and deadlines. All were dismissed as lacking evidence or sufficient effect on the final tally.

In 2000, the Supreme Court’s decision in Bush vs. Gore essentially decided the close election.

Most election-law experts today are not predicting the winner of the 2024 election will be decided in the courts.

“But in this climate, with so many states where the outcome could be very close, you couldn’t absolutely rule it out,” said Jessica Marsden, a voting-rights attorney at the nonprofit organization Protect Democracy.

Here’s a look at some issues causing waves in key swing states that could end up at the Supreme Court if there is a razor thin margin separating Vice President Kamala Harris and Trump.

Pennsylvania mail-ballot dates

In Pennsylvania, ballots sent by mail will go uncounted if voters do not write a date with their signature on the envelope, even if it arrives well before election day.

Officials in some counties say they will try to notify voters of their error so they can fix it in time.

Two weeks ago, the Pennsylvania Supreme Court refused to rule on mail-ballot errors, leaving the state’s 67 counties to decide on how to proceed.

“It’s a confusing and evolving legal situation,” said Philip Hensley-Robin, executive director of Common Cause Pennsylvania.

Pennsylvania could produce the most significant legal battle if only a few thousand votes separate the candidates.

Four years ago, more than 30,000 Pennsylvanians made ballot errors when the state turned to voting by mail because of the COVID-19 pandemic.

In the primary election in April, about 8,500 mail ballots, or slightly more than 1%, were rejected by counties because of an error on the envelope, according to state data.

State officials are confident the percentage of ballot errors this year will be lower — but not zero. “Pennsylvania is still the one to watch” because of the legal uncertainty, Marsden said.

The ACLU of Pennsylvania had gone to court this year arguing these otherwise valid votes should not be discarded. A state appeals court agreed in August and held that it violated the state Constitution to throw out ballots because of “meaningless and inconsequential paperwork errors.”

The Republican National Committee also had sued arguing the “notice and cure” policies adopted by some counties violated the state law on mail ballots adopted in 2020.

In a pair of rulings, the state Supreme Court rejected both claims and said it would not impose or allow “substantial alterations to existing laws or procedures” in the run-up to the election.

This decision to step back was widely seen as a reaction to what happened four years ago. Then, the Pennsylvania justices were sharply criticized by Republicans and the Supreme Court’s conservatives for having extended by three days the deadline for counting late-arriving mail ballots.

The state justices, siding with Democrats, pointed to delays in the mail caused by the pandemic.

When the Republicans appealed the three-day extension for counting ballots, the Supreme Court upheld the state decision on a 4-4 tie vote. It was a week before Justice Amy Coney Barrett, Trump’s third appointee, won confirmation.

Chief Justice John G. Roberts Jr. voted with the three liberals while four conservatives voted to grant the GOP appeal.

The outcome of the hard-fought court battle had little consequence, however, because there were only a few thousand late-arriving ballots and President Biden won the state by about 80,000 votes.

This year, Pennsylvania voters must return their mail ballots by election day if they are to be counted.

But if the race is a near-deadlock in Pennsylvania, both sides could muster legal arguments over counting or not counting mail ballots with minor errors.

The ACLU says federal civil rights law forbids denying the right to vote because of an “error or omission” that is “not material” to whether the voter is qualified.

Republicans could argue that it is unfair for counties to adopt different rules for fixing ballots with small errors.

The Bush vs. Gore decision that ended the presidential race in 2000 is in the small category of landmark Supreme Court rulings that are almost never cited by lawyers or judges.

But the justices in their unsigned opinion said it violated the principle of “equal protection of the laws” for Florida counties to use different standards for counting or not counting punch card ballots.

Nevada late-arriving ballots

In Nevada, the issue is late-arriving mail ballots.

The RNC and the Trump campaign sued in May arguing that federal law sets a single election day, and thereby forbids counting late ballots.

A judge dismissed the suit, which itself came very late.

California and and a dozen other states have similar laws.

The Trump campaign could try to revive the issue in a long-shot appeal to the conservative Supreme Court if a few thousand votes are crucial in Nevada.

Overseas ballots

Two weeks ago, the RNC sued in Michigan and North Carolina over what it called “illegal overseas voting.”

Its lawyers said these two states had authorized Americans living abroad to cast votes in their states, even if they were not residents.

RNC Chairman Michael Whatley said “North Carolinians and Michiganders should not have their votes canceled by those who have never lived in the state in the first place.”

The RNC said these overseas ballots, many of which come from members of the military, should be set aside and counted separately.

Five GOP lawmakers in Pennsylvania filed a similar claim.

These lawsuits, like the one in Nevada, appear to be aimed at a possible postelection appeal.

“They read more like press releases than lawsuits,” said Wendy Weiser, vice president for democracy at the Brennan Center. “They may go nowhere in court but they look like they are ginned up to challenge the outcome after the election.”