The justices left in place a state Supreme Court ruling that elections officials must count provisional ballots cast by voters whose mail-in ballots were rejected.
The ruling is a victory for voting-rights advocates, who had sought to force counties - primarily Republican-controlled counties - to let voters cast a provisional ballot on Election Day if they had realized that their mail-in ballot was to be rejected for various garden-variety errors.
RELATED | Long lines, some confusion on final day for 'on demand' voting in Bucks County
Long lines, some confusion on final day for 'on demand' voting in Bucks County
As of Thursday, about 9,000 ballots out of more than 1.6 million returned have arrived at elections offices around Pennsylvania lacking a secrecy envelope, a signature or a date, according to state records.
Pennsylvania is the biggest presidential election battleground this year, with 19 electoral votes, and is expected to play an outsized role in deciding the election between Republican Donald Trump and Democrat Kamala Harris.
It was decided by tens of thousands of votes in 2016 when Trump won it and again in 2020 when Democrat Joe Biden won it.
The ruling comes as voters had their last chance Friday to apply for a mail-in ballot in a bellwether suburban Philadelphia county while a county clear across the state gave voters who didn't receive their ballot in the mail another chance to get one.
A judge in Erie County, in Pennsylvania's northwestern corner, ruled Friday in a lawsuit brought by the Democratic Party that about 15,000 people who applied for a mail ballot but didn't 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. But the judge's 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 didn't receive in the mail, and get another one over the counter.
In suburban Philadelphia's Bucks County, a court set a deadline of 5 p.m. for voters there to apply for and receive a mail-in ballot.
Lines outside the county's elections office in Doylestown were long throughout the day - snaking down the sidewalk - with the process taking about two hours by Friday afternoon.
Nakesha McGuirk, 44, a Democrat from Bensalem, sized up the line and said: "I did not expect the line to be this long. But I'm going to stick it out."
She faces a long work commute next week and worried about her ability to make it to the polls on Election Day. "I figured that rather than run into the risk of not getting home in time to go and vote, that it would be better to just do it this way early," said McGuirk, a Harris supporter.
Republican voter Patrick Lonieski, a Trump supporter from Buckingham, also found it more convenient with his work schedule to vote Friday in a county he called "pivotal" to the outcome.
"I just want to make sure I get my ballot in and it's counted," said Lonieski, 62, who was joined by his 18-year-old son, voting for the first time.
The line steadily dwindled as 5 p.m. approached. One last straggler broke into a run to make it by the deadline as elections workers cheerfully counted down the seconds. "Let's go! Hurry up! You can do it!" a bystander yelled. People broke into applause as she walked through the door - just in time.
A Bucks County judge had ordered the three-day extension in response to a Trump campaign lawsuit alleging that 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.