Pennsylvania voters will be able to cast a provisional vote if they make an error and forget to put their mail-in vote in a required secrecy envelope, the US supreme court ruled on Friday, a decision that could lead to thousands more votes being counted in a key battleground state where the presidential race is extremely tight.

The supreme court announced its decision on Friday on its emergency docket, giving no reasoning for its ruling, which is customary in emergency cases.

Republicans had asked the court to intervene after the Pennsylvania supreme court ruled 4-3 earlier this week that voters should not be disenfranchised if they forget the secrecy envelope. Voters in Pennsylvania must place their ballots in two envelopes: an inner envelope that ensures its secrecy, and an outer envelope that they mail.

Pennsylvania law does not allow for a voter to cast a provisional ballot – an emergency vote at the polls – if they have cast a timely mail-in vote. But a majority of justices on the state supreme court said that a ballot that was rejected because it lacked a secrecy envelope did not count as a cast ballot. “Because electors failed to comply with the mandatory secrecy envelope requirement, they failed to cast a ballot,” the state supreme court wrote.

The scope of the ruling is not immediately clear. As of Thursday, nearly 9,000 out of 1.6m mail-in ballots returned had arrived without a secrecy envelope, the Associated Press reported. The NBC News decision desk estimated that thousands of votes could be affected in a state that could come down to extremely fine margins.

“In Pennsylvania and across the country, Trump and his allies are trying to make it harder for your vote to count, but our institutions are stronger than his shameful attacks. Today’s decision confirms that, for every eligible voter, the right to vote means the right to have your vote counted,” read a joint statement from Kamala Harris’s campaign and the Democratic National Committee.

Pennsylvania adopted a law in 2019 that allows all voters to cast a mail-in ballot. But the law is not clear when it comes to whether officials should be required to notify the voter of technical errors on their ballots – a missing secrecy envelope, signature or date – and help them fix them.

Thirty-eight counties in the state notify voters of errors and give them the opportunity to fix it, according to an analysis by Votebeat. But the court’s decision on Friday means that any voter in Pennsylvania who has a ballot rejected on a technicality will have the chance to cast a provisional ballot on election day to be counted.

The dispute stemmed from a special election earlier this year in which two voters in Butler county, near Pittsburgh, had their mail-in ballots rejected, were invited to cast a provisional ballot, and then had their provisional ballots rejected.

Justice Samuel Alito wrote on Friday that the case before the court was relatively narrow, and that the court’s intervention at this stage would not prohibit any other Pennsylvania county from prohibit provisional voting. Still, he expressed openness to the argument from Republicans that the state supreme court may have exceeded its authority.

Alito’s statement, joined by Clarence Thomas and Neil Gorsuch, was significant because he seemed to suggest that the court could use the case to revisit the question of how far state supreme courts can go in policing election laws.

The supreme court said last year that state courts can police election rules, but that sometimes they can go too far. It has declined to clarify the ambiguity of its ruling.

Pennsylvania is the biggest presidential election battleground this year, with 19 electoral votes. Donald Trump won the state in 2016, then lost it in 2020.

A judge in Erie county, in Pennsylvania’s north-western corner, ruled on Friday in a lawsuit brought by the Democratic party that about 15,000 people who applied for a mail ballot but did not receive it may go to the county elections office and get a replacement through Monday.

The deadline to apply for a mail-in ballot has passed in Pennsylvania, the biggest presidential battleground this year and a state that has hosted far more visits by Trump and Harris, his Democratic rival, than any other.

The ruling means that Erie county’s elections office will be open every day through Monday for voters to go in, cancel the mail-in ballot they did not receive in the mail, and get another one over the counter, said Cliff Levine, a lawyer for the state Democratic party.

In suburban Philadelphia’s Bucks county, a court set a deadline of 5pm for voters there to apply for and receive a mail-in ballot.

Lines outside the elections office in Doylestown were long throughout the day – snaking down the sidewalk – with the process taking about two hours by Friday afternoon.

A Bucks county judge had ordered the three-day extension in response to a Trump campaign lawsuit alleging voters faced disenfranchisement when they were turned away by county application-processing offices that had struggled to keep up with demand, leading to frustration and anger among voters.

The Trump campaign lawsuit said people who were in line by Tuesday’s 5pm deadline to apply in person for a mail ballot should have been allowed to get a ballot, even after the deadline. However, Bucks county’s election office denied voters that right and ordered them to leave, the lawsuit said.

Bucks county judge Jeffrey Trauger ruled that the board of elections violated the state election code and ordered an extension through Friday.

Unlike other states, Pennsylvania does not have true early in-person voting. Voters can apply early for mail ballots online or in person at county election buildings.

Doing so in person can take about 12 minutes and requires applying for a mail ballot, waiting for a bar-coded envelope to be printed and then, if voters wish, they can cast the ballot on the spot. Or they can put it in a drop box or a mailbox. Election offices must receive the ballots by 8pm on Tuesday.