HARRISBURG, Pa. (AP) — 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 order is a win for McCormick and a loss for Casey as the campaigns prepare for a statewide recount and press counties for favorable ballot-counting decisions. The Democratic-majority high court's order reiterates the position it took previously that the ballots shouldn't be counted in the election, a decision that three Democratic-controlled counties nevertheless have challenged.

The Associated Press called the race for McCormick last week, concluding that not enough ballots remained to be counted in areas Casey was winning for him to take the lead.

As of Monday, McCormick led by about 17,000 votes out of almost 7 million ballots counted — inside the 0.5% margin threshold to trigger an automatic statewide recount under Pennsylvania law.

Republicans last week had asked the court to bar counties from counting the ballots, saying those decisions violate both the court’s recent orders and its precedent in upholding the requirement in state law that a voter write the date on their mail-in ballot's return envelope.

Democratic-majority election boards in Montgomery County, Philadelphia and Bucks County voted to count the ballots that lacked a correct date, echoing election officials around the state who say the date tells them nothing about a voter’s eligibility or a ballot’s legitimacy.

Republicans maintain that the date is a critical element of ballot security.

At first, the GOP also asked the court to block the count in Centre County, but later said the county had only counted three ballots that GOP officials found to be reasonable.

Statewide, the number of mail-in ballots with wrong or missing dates on the return envelope could be in the thousands. However, most counties — including several heavily populated counties controlled by Democrats — didn't count them.

Democrats cast more mail-in ballots than Republicans, and Democrats in the past have supported counting ballots that trip over what they view as meaningless clerical requirements in state law.

Various courts have ruled against the dating requirement in at least a half-dozen cases — including once by the 3rd U.S. Circuit Court of Appeals — but higher courts have always reinstated it.

Meanwhile, the state Supreme Court has put off ruling on a pending case that calls into question whether the law violates the constitutional right to vote.

McCormick took a position aligned with Democrats in his failed eleventh-hour bid to close the gap in votes with celebrity heart surgeon Dr. Mehmet Oz in Pennsylvania’s Republican primary contest for U.S. Senate.

In that case, McCormick’s lawyer told a state judge that the object of Pennsylvania’s election law is to let people vote, “not to play games of ‘gotcha’ with them.”

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