In more than two hours of oral arguments Monday, the U.S. Supreme Court debated whether federal law allows states to count mail-in ballots received after Election Day.

The Supreme Court heard arguments on mail-in ballots received by officials after they are postmarked on Election Day.

Because more than a dozen states have similar laws, the court’s ruling – which is expected by late June or early July – could have significant implications for federal elections, beginning as soon as ...