The U.S. Supreme Court heard oral arguments in a case that hinges on the meaning of Election Day — and it’s got Democrats sweating.

The case, Watson v. Republican National Committee, challenges a Mississippi law that allows mail-in ballots received long after Election Day to be counted.

It’s one of 14 states, including populous ones like California and New York, that allows a post-Election Day “grace period” (five business days in Mississippi but longer elsewhere) for ballots to be returned.

That law is being challenged by the RNC.

Taking a states’ rights stance, Mississippi’s defenders argue that deciding when ballots must arrive is a state’s decision.

The U.S. Constitution reserves to state legislatures the authority to prescribe “the times, places, and manner of holding federal elections.”

True ­– but the Constitution empowers Congress to set the date.

In 1824, Congress chose the Tuesday after the first Monday in November as Election Day, and it’s been the same ever since.

But in recent years the meaning of Election Day has been eroded — its finality and certainty upended by state laws allowing absentee ballots to be counted days later.

At stake is a national treasure of civic virtue, long understood as the one day on which the whole electorate’s votes get counted, and everyone knows the results.

The justices’ decision will have national impact and could potentially affect November’s midterms.

New York’s law allows a ballot postmarked by Election Day to be counted as many as seven days later — and worse yet, it deems a ballot lacking a postmark to “be presumed to have been timely mailed or delivered.”

That’s crazy: It invites cheating.

And many, including House Speaker Mike Johnson, suspect that as ballots straggle in, fraudulent efforts to change the outcome could be afoot.

Public confidence in the honesty of our elections matters, and states should avoid any procedure that undermines it.

For at least one of the justices weighing the Mississippi case, that will be a major concern.

In 2020, when the court was asked to intervene in a conflict between Pennsylvania’s highest court and its state legislature over a similar question, Justice Brett Kavanaugh said he favored limiting ballot-counting to Election Day.

Doing so, he wrote, would “avoid the chaos and suspicions of impropriety that can ensue if thousands of absentee ballots flow in after election day and potentially flip the results of the election.”

Kavanaugh’s concern will likely influence the deliberations: Justice Samuel Alito voiced similar worries.

Much of the discussion, though, focused on the history and meaning of Election Day, which Mississippi’s defenders claimed merely means the day on which voters decide, not the day their votes are counted.

That’s historically implausible, but the three leftists on the bench — Justices Elena Kagan, Sonia Sotomayor and Ketanji Brown Jackson ­– showed interest in that theory.

Democrats argue that striking down the Mississippi grace period would throw the coming midterms into chaos.

Even if New York and other delayed-count states wait for the court to rule before acting, they’ll still have more than five months to alert their voters about the need to mail their ballots in early.

It’s hardly an insurmountable challenge. Getting to the mailbox with plenty of time to spare is a small price to pay for living in a representative democracy.