John Fetterman's campaign demanded that mail-in ballots be counted in upcoming races in the state.
A provision of a state law requiring mail-in ballots to include the date is alleged to have been used in the lawsuit.
The legal action came after the Pennsylvania Supreme Court ordered election officials to disqualify mail-in and Absentee ballots with missing dates. The judges decided on the basis of the Pennsylvania Election Code, which requires voters to fill out and sign a declaration when they cast their ballot.
Thousands of mail-in ballots have been deemed invalid due to missing or incorrect dates. When voters enter an incorrect date, such as their birthdate, instead of the date they completed or signed their ballot, election officials will reject them.
The date instruction is not relevant to determining voter eligibility according to the legal filing. They said that if a marked date is needed to prove voter qualifications, it should be the day of the election.
According to the suit, one of the voters involved in the case has a medical condition that makes her blind and unable to cast a ballot in person.
The Pennsylvania Supreme Court was asked to look at a Petition for Discretionary Review to see if the date instruction in the state's election code should be read as directory or mandatory. The majority of justices ruled that a ballot's failure to comply with the stipulations means it should not be tabulated.