The unanimous decision came after officials in at least four counties had ignored the court’s pre-election guidance not to count mail ballots that were undated or misdated.

Several Pennsylvania county election boards voted to include illegal ballots in ballot recount for Senate seat

Pennsylvania's state Supreme Court on Monday weighed in on a flashpoint amid ongoing vote counting in the U.S. Senate election between Democratic Sen. Bob Casey and Republican David McCormick, ordering counties not to count mail-in ballots that lack a correct handwritten date on the return envelope.

The Pennsylvania Supreme Court has directed county officials not to count mail ballots for the general election that arrived on time but in envelopes without current dates handwritten by voters.