WASHINGTON — Rejecting a Republican National Committee challenge, the Supreme Court ruled Monday that elections officials may count mail-in ballots that arrive after Election Day if they were postmarked beforehand. The court, divided 5-4, held that the Mississippi law challenged by the RNC does not unlawfully conflict with the federal law that sets Election Day in early November. The ruling, authored by Justice Amy Coney Barrett, is a setback for President Donald Trump, who has frequently criticized mail-in voting, claiming without offering evidence that it is rife with fraud. Two of the court’s conservatives were joined by the three liberals...