At a board meeting on Monday night, City Commissioners voted not to count 607 undated or incorrectly dated mail ballots.
The decision complies with Monday's ruling by Pennsylvania's State Supreme Court.
The ruling is a win for Republican challenger Dave McCormick and a loss for incumbent Democratic Sen. Bob Casey as the campaigns prepare for a statewide recount and press counties for favorable ballot-counting decisions while election workers are sorting through thousands of provisional ballots.
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 Republicans say several Democratic-controlled counties nevertheless challenged.
In a statement, Gov. Josh Shapiro, a Democrat, said a lack of legal clarity had surrounded the ballots, putting county officials in a position where they were "damned if they did and damned if they didn't - likely facing legal action no matter which decision they made on counting."
It comes amid a gust of fresh litigation in recent days filed by both campaigns, contesting the decisions of about a dozen counties over whether or not to count thousands of provisional ballots.
Casey's campaign says the provisional ballots shouldn't be rejected for garden-variety errors, like a polling place worker forgetting to sign it. Republicans say the law is clear that the ballots must be discarded.
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.
Statewide, the number of mail-in ballots with wrong or missing dates on the return envelope could be in the thousands.
The Associated Press called the race for McCormick last week, though Casey - who is behind by more than 17,000 votes - has not conceded.
ABC News has not yet projected a winner in this race.
The Associated Press contributed to this report.