WASHINGTON — The Supreme Court said on Thursday that election officials in Pennsylvania may count mailed ballots accompanied by voters’ declarations that were signed but not dated. The court’s order came in a tight race for a seat on a state court, but it is likely to affect other contests in the state as well.

The court’s brief order gave no reasons, which is typical when the justices act on emergency applications.

The court’s three most conservative members — Justices Samuel A. Alito Jr., Clarence Thomas and Neil M. Gorsuch — dissented, saying that the court should address the issue presented in the case in time for the November elections.

The case concerned a state law enacted in 2019 that permitted all registered voters to vote by mail. The law required voters using mailed ballots to “fill out, date and sign” a declaration printed on the outside of the return envelope that said they were qualified to vote.

The Supreme Court’s order let stand a ruling from a unanimous three-judge panel of the U.S. Court of Appeals for the Third Circuit, in Philadelphia, which said the part of the state law requiring the declarations to be dated ran afoul of a provision of the federal Civil Rights Act of 1964. The provision prohibits government officials from denying the right to vote “because of an error or omission” if it “is not material in determining whether such individual is qualified under state law to vote.”

The case arose from an election in November 2021 for a seat on the Lehigh County Court of Common Pleas in which David Ritter, a Republican, leads Zachary Cohen, a Democrat, by 71 votes. The local elections board determined that it would also count 257 undated ballots, and challenges in state and federal court followed.

It was undisputed that the undated ballots were received by Election Day and that the elections board had accepted ballots with incorrect dates, including birth dates, rejecting only missing ones.

The suit, brought by five voters who had submitted undated ballots, argued that the federal law required that all 257 undated ballots be counted. The voters, represented by the American Civil Liberties Union, told the Supreme Court that requiring a date served no purpose.

“The handwritten date is so inconsequential that the Board of Elections accepted ballots where voters wrote any date whatsoever on the return envelope, even dates from decades ago,” the voters’ brief said. “The county clerk affirmed he would have accepted envelope dates from the future. Yet voters who mistakenly omitted the envelope date were disenfranchised.”

In March, Judge Joseph F. Leeson Jr. of the Federal District Court in Allentown, Pa., rejected the suit, saying that only the attorney general was authorized to sue under the federal law.

The Third Circuit disagreed, saying that the voters were entitled to sue and that the requirement of a dated declaration did not help determine whether the voter was qualified. “The requirement is material if it goes to determining age, citizenship, residency or current imprisonment for a felony,” Judge Theodore A. McKee wrote, concluding that adding a date to a signature did not aid in those determinations.

Justice Alito, writing for the three dissenters, said the federal law did not appear to address the requirement that voters date their declarations.

“When a mail-in ballot is not counted because it was not filled out correctly, the voter is not denied ‘the right to vote,’” he wrote. “Rather, that individual’s vote is not counted because he or she did not follow the rules for casting a ballot.”

Justice Alito added that the dating requirement was little different from “rules setting the date of an election, the location of the voter’s assigned polling place” or “the address to which a mail-in ballot must be sent.”

“One may argue that the inclusion of a date does not serve any strong purpose and that a voter’s failure to date a ballot should not cause the ballot to be disqualified,” he wrote.

But the federal law, he added, “applies only to errors or omissions that are not material to the question whether a person is qualified to vote. It leaves it to the states to decide which voting rules should be mandatory.”

In his emergency application, Mr. Ritter urged the Supreme Court to rule promptly.

“Election contests and automatic recounts are happening now, as are preparations for the general election in November,” Mr. Ritter’s lawyers wrote, adding, “All relevant stakeholders — election officials, political parties, candidates, campaigns and voters — need resolution from this court.”