The Republican National Committee urged the justices to block a state court ruling that allows people to vote provisionally when election officials identify problems with their mail ballots - specifically, when they fail to put them into a "secrecy" envelope before sending them off in the mail.
Those ballots, because they are missing the secrecy sleeve, are referred to as "naked ballots."
"This case is of paramount public importance, potentially affecting tens of thousands of votes in a state which many anticipate could be decisive in control of the U.S. Senate or even the 2024 presidential election," the RNC said in its appeal.
The case arrived hours after Virginia asked the Supreme Court to allow the state to continue a program to remove suspected noncitizens from the voter rolls. Both cases are likely to be handled with remarkable speed - by Supreme Court standards - and could yield orders within days.
The RNC asked the Supreme Court to decide the Pennsylvania case by Friday.
Republicans claimed in their emergency appeal that Pennsylvania's highest court usurped power reserved to the state legislature by allowing voters to cast provisional ballots after their mail-in ballots were tossed out.
"When the legislature says that certain ballots can never be counted, a state court cannot blue-pencil that clear command into always," the RNC wrote.
The state Supreme Court, the party said, effectively allowed voters to fix or "cure" defective ballots, "a process everyone agrees the General Assembly has deliberately chosen not to create."
US Supreme Court Justice Samuel Alito, who handles emergency cases arising from Pennsylvania and other mid-Atlantic states, requested a response in the case by Wednesday afternoon.
The initial lawsuit was filed by two voters in Butler County, north of Pittsburgh, who initially submitted mail-in ballots. But they didn't enclose their ballots in the secrecy sleeve before placing them in an outer mailing envelope. A machine scanner identified that problem and the voters received an automated message alerting them that their vote would not be counted.
The voters then showed up at their precincts during the state's primary election and attempted to cast provisional ballots but learned those votes wouldn't be counted, either. County election officials said state law barred them from voting because they had already submitted a mail-in ballot, even though that first ballot was deemed defective.
Pennsylvania's top court sided with the voters, ruling that election officials were correct to toss the mail-in ballots but "erred in refusing to count" their provisional ballots. Pennsylvania law, the state's high court ruled, requires that provisional ballots "be counted if there were no other ballots attributable to the electors. There were none."
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Initially a dispute between the voters and Butler County, both the Republican National Committee and the Democratic National Committee intervened. Democrats sided with the voters, while Republicans backed the county.
It is the Republican Party that has appealed the state court's decision to the US Supreme Court.
Though the latest appeal has added another election case to the Supreme Court's docket, it's not clear that any of the pending appeals will have a dramatic effect on the outcome of the race between former President Donald Trump and Vice President Kamala Harris.
In the Pennsylvania case, it's unclear how many ballots the dispute will affect.
New York University law professor Richard Pildes, in a recent blog post, offered a back-of-the-envelope estimate that the case may affect between 400 and 4,000 ballots. However, his analysis focused on the secrecy envelope issue.
Both the RNC and its opponent in the case, the American Civil Liberties Union of Pennsylvania, are interpreting the ruling to apply to other technical defects on a mail-in ballot, such as a missing signature or date, that would prompt the ballot's disqualification.
The party is asking the high court to pause the state court's ruling or, alternatively, to separate the provisional ballots so they can be challenged after the election.
Republicans are partly relying on a legal theory known as the independent state legislature doctrine, which suggests that the US Constitution gives state legislatures virtually unchecked power to set election rules.
The Supreme Court grappled with and ultimately declined to embrace that conservative-backed theory in a case last year involving North Carolina's congressional maps.
But while the 6-3 opinion from Chief Justice John Roberts didn't go as far as some Republicans had hoped, it also didn't close the door on the US Supreme Court reviewing future state high court rulings dealing exclusively with state election laws.
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