The Pennsylvania Supreme Court on Monday ordered election officials in the state to stop counting mail-in ballots marked with the wrong date or missing dates from their outer envelopes.

The court order specifies that Bucks, Montgomery and Philadelphia counties, where Republicans argue that officials have opted to count mail-in ballots with errors on their outer envelopes, must adhere to the high court’s earlier rulings, which said undated or misdated mail-in ballots should not be counted.

The directive is a courtroom victory for Republican Dave McCormick, who holds a narrow lead over Democratic Sen. Bob Casey in a razor-thin Senate race that is headed to a recount this week.

Elizabeth Gregory, a spokesperson for McCormick’s campaign, called the ruling “a massive setback to Casey’s attempt to count illegal ballots” in a post on X, adding that McCormick “looks forward to taking the Oath of Office in January.”

Tiernan Donohue, a campaign manager for Casey, characterized the litigation in a statement Monday as part of an effort by McCormick and other Republicans to disenfranchise Pennsylvania voters.

“David McCormick and the national Republicans are working to throw out provisional ballots cast by eligible Pennsylvania voters and accepted by county boards. It is wrong and we will fight it,” Donohue said.

McCormick declared victory Friday after The Associated Press projected him the winner. NBC News has not yet projected a winner in the race, which remains too close to call. McCormick leads Casey by 17,408 votes with 99.7% of the vote in and 24,000 ballots still to be counted.

Pennsylvania rules trigger a recount of ballots when the margin is less than 0.5 percentage points. The recount, set to begin this week, must be completed by noon Nov. 26.

The Republican National Committee and the Republican Party of Pennsylvania had filed the petition seeking a court order, singling out election boards led by Democrats in Bucks, Montgomery and Philadelphia counties that had previously voted to tabulate ballots lacking correct dates.

The counties had decided to count those ballots under the reasoning that an incorrect date did not indicate that a voter was ineligible, nor did it suggest that the ballot was illegitimate.

The Pennsylvania Supreme Court on Monday issued a ruling reiterating its previous stance that undated or misdated mail ballots should not be counted in the 2024 election, dealing a blow to Democratic U.S. Sen. Bob Casey’s hopes that a recount and litigation will help him overcome his more than 15,000-vote deficit to Republican Dave McCormick.

The 4-3 ruling, which was requested by the Republican Party and opposed by Casey’s campaign, followed moves by elections officials in Democratic-controlled counties — including Philadelphia, Bucks, and Montgomery — to have the ballots counted despite the high court instructing them to exclude those votes earlier in the year. The ruling applies to all counties.

» READ MORE: Undated mail ballots won’t be counted in next week’s election, Pa. Supreme Court rules

Democrats in those counties and elsewhere have pushed to include mail ballots with defects related to the dates voters are required to write on them because the dates are not used by election administrators to determine whether ballots are legitimate. Instead, they only count ballots that are received between when the ballots are distributed and Election Day, making it impossible for a vote to be counted outside of that timeframe regardless of what date a voter writes on the ballot.

Republicans have argued that those votes must be excluded from the count because state law requires voters to date their mail ballots. McCormick’s campaign joined the GOP lawsuit after it was filed.

While the ruling settles how these types of ballots are handled this year, the longer legal battle may not be over because the court has not yet weighed in on the underlying question of whether rejecting undated ballots on what Democrats describe as a technicality constitutes a violation of rights guaranteed to voters by the state constitution.

In a ruling issued shortly before Election Day, the Pennsylvania Commonwealth Court found that it did, though that case centered on a special election held in Philadelphia earlier this year. The state Supreme Court stayed the lower court’s decision before Election Day, deciding at the time that it was too close to the Nov. 5 vote for any last-minute changes to rules surrounding which votes should be counted.

Democratic Justices David Wecht and Kevin Dougherty were joined by Republican Justices Kevin Brobson and Sallie Updyke Mundy in the majority decision Monday. Democratic Justices Debra Todd, Christine Donohue, and Daniel McCaffery dissented.

The total number of ballots in question is likely well under 10,000 and would not be enough to erase Casey’s deficit alone. But the three-term incumbent is also in legal fights with McCormick’s team over how various counties have handled certain categories of provisional ballots across the state.

The Associated Press has called the race for McCormick, but Casey has declined to concede.

Casey campaign manager Tiernan Donohue said Monday that the Democrat wants to ensure all legitimate votes are counted and is being opposed by McCormick’s campaign efforts to “disenfranchise” Pennsylvanians.

“Senator Casey is fighting to ensure Pennsylvanians’ voices are heard and to protect their right to participate in our democracy – just like he has done throughout his entire career,” Casey campaign manager Tiernan Donohue said. “Meanwhile, David McCormick and the national Republicans are working to throw out provisional ballots cast by eligible Pennsylvania voters and accepted by county boards.”

McCormick spokesperson Elizabeth Gregory cast the ruling as a “massive setback to Senator Casey’s attempt to count illegal ballots.”

“Bucks County and others blatantly violated the law in an effort to help Senator Casey,” Gregory said. “Senator-elect McCormick is very pleased with this ruling and looks forward to taking the Oath of Office in a few short weeks.”

Staff writer Jeremy Roebuck, Gillian McGoldrick, Katie Bernard, and Fallon Roth contributed to this article.