Pennsylvania Democrats are increasingly alarmed that the U.S. Supreme Court might intervene to throw out mail ballots that arrive after Election Day, after three conservative justices on the high court this week held open the possibility of reversing a state court ruling that extended the deadline for returning them.

Meanwhile, several Pennsylvania counties have said they won’t begin counting mail ballots until the day after Election Day, increasing the likelihood that early returns that night could give a misleading impression of the vote, since Democrats are voting far more heavily by mail. In some counties, their votes may not begin being tallied until Nov. 4.

And the prospect of a long delay before a fuller picture of the results is known, Democrats fear, will open a window for Trump to prematurely declare victory as public perceptions harden around incomplete tallies.

Some Pennsylvania Democrats sound increasingly confident Joe Biden will have the votes to win the battleground state — and with it, likely the presidency.

But one of Biden’s closest allies in Pennsylvania worried Thursday that an unrestrained President Donald Trump or a newly reshaped Supreme Court could dismiss or thwart ballots voters had cast under the current deadline, and potentially secure a second term for the president.

“That’s my number-one concern,” Sen. Bob Casey, one of Pennsylvania’s most senior Democrats and, like Biden, a Scranton native, said in an interview. “All of these ballot issues, whether it’s by way of litigation or by way of the president engaging in conduct that might be unlawful. He’s not constrained by the law, he’s not constrained by convention or deference to institutions.”

The U.S. Supreme Court, in deciding Wednesday not to fast-track a Republican effort to strike down the state Supreme Court’s ruling extending the deadline by three days, did not address the merits of the case — only whether it could be decided before Election Day.

“There is a strong likelihood that the State Supreme Court decision violates the Federal Constitution,” Justice Samuel Alito wrote in a statement joined by Justices Clarence Thomas and Neil Gorsuch.

Casey said Thursday it would be “a travesty” and "potentially undermine the will of the people” if votes are thrown out after the election, when voters acted in good faith in following the new deadline set by the state Supreme Court.

“If someone fills out a ballot and puts it in a secrecy envelope ... and it’s postmarked on Election Day ... it’s my view they have voted," he said.

Trump is consistently trailing in polls in Pennsylvania, but he has repeatedly attacked voting in the state with false accusations, particularly in Philadelphia. He has also frequently said voters should know the winner on election night, even though millions of mail ballots nationwide likely won’t be counted yet — including military and overseas ballots in Pennsylvania that can continue arriving up to a week after Election Day.

He has refused to commit to a peaceful transition of power if he loses.

“A 3 day extension for Pennsylvania is a disaster for our Nation, and for Pennsylvania itself,” Trump tweeted Thursday. He continued raising baseless claims of widespread voter fraud. “The Democrats are trying to steal this Election. We have to get out and VOTE in even larger numbers. The Great Red Wave is coming!!!”

State Sen. Sharif Street, a Philadelphia Democrat who is vice chair of the state party, said he expects “theatrics” and legal challenges no matter what.

“I fully expect the Democratic Party will be in court with Donald Trump the next day," Street said. "I fully expect he will allege fraud that hasn’t taken place and victories he hasn’t won.”

The questions about the speed and fairness of the vote count come as the election barrels into its final days, and with memories of 2016 seared into Democrats' minds. Trump that year won Pennsylvania, Michigan, and Wisconsin by less than 1 percentage point each, securing the presidency by a razor thin margin.

Along with being potentially the decisive state in the presidential race, Pennsylvania has also become one of the country’s central legal battlegrounds over voting rights.

Much of the attention this month has focused on GOP efforts to restore the normal mail ballot deadline, which under state law requires they be received by 8 p.m. on Election Day, Nov. 3.

The state Supreme Court, dominated by Democrats, extended the deadline to the Friday after the election. Republicans have tried to strike down that extension at the U.S. Supreme Court, failing at first when the court split 4-4.

In the second attempt, the high court on Wednesday declined to take up the new challenge on an expedited basis. But three conservative justices suggested they might revisit the issue after the election, and possibly retroactively dismiss mail ballots that arrive after Nov. 3. And newly confirmed Justice Amy Coney Barrett now gives conservatives a 6-3 majority on the court.

Sen. Pat Toomey, a Pennsylvania Republican, said it was the state court’s “unconstitutional” deadline extension that threatens Pennsylvanians' votes.

“The U.S. Supreme Court could very well decide it must overturn the Pennsylvania Supreme Court’s unlawful ruling. That could result in disqualifying ballots that arrive after Election Day,” Toomey said. He urged voters to deliver ballots to a drop box, an elections office, or vote in person “to help ensure the Pennsylvania Supreme Court’s illegal action does not cost them their vote.”

Almost 3.1 million Pennsylvania voters have submitted applications for mail ballots, and about 2.1 million have returned them to county election offices, according to state data released Thursday.

About 63% of the applications have been from Democrats, 25% by Republicans, and 12% by unaffiliated voters.

That imbalance has fueled Democratic concern that in a neck-and-neck election, throwing out even a fraction of mail ballots could tilt the state to Trump. State officials have directed county elections offices to separate ballots that arrive after Election Day, in case the order is to reject them.

Some Democrats, however, expressed confidence that the count will go smoothly and noted that only a small fraction of votes could be called in question by this challenge. (Though there may also be other GOP attempts to disqualify votes).

Gov. Tom Wolf and Secretary of State Kathy Boockvar said Thursday that voters can be confident they’ll be able to vote safely, and that every vote will be counted.

But they also acknowledged they’re still scrambling to ensure voters understand how to properly submit mail ballots. Boockvar has urged voters at this point to return ballots to drop boxes or vote in person on Election Day, not to mail them nor depend on the deadline extension.

It would take very specific — but possible — circumstances for the court to sway the presidential election by dismissing Pennsylvania votes.

The national race would have to be so close that changing Pennsylvania’s result would change the victor. And Pennsylvania would have to be so close that throwing out a fraction of mail ballots would flip its result. Several legal experts have said it’s unlikely the high court would retroactively dismiss votes cast under rules that were in place.

But even if most votes arrive on time, the question of how they are counted provides another cause for anxiety. With a deluge of mail ballots expected, Wolf and Boocvkar acknowledged that counting could be protracted.

State law in Pennsylvania bars election officials from even opening mail ballots before Election Day, leaving them with much work to do — checking signatures, opening outer envelopes, inner envelopes, flattening the ballots, and scanning them — on Nov. 3 and after.

If it’s a particularly close race, as in 2016, it could be several days before it’s clear who won.

And a handful of the state’s 67 counties, some citing staffing limits and the demands on Election Day, restrictions, won’t begin counting mail ballots until the day after.

In five such counties — Beaver, Cumberland, Mercer, Monroe, and Montour — voters have requested more than 155,000 mail ballots. Of those, about 58% have been requested by Democrats, 30% by Republicans, and 13% by people unaffiliated with either party.

If 60% of registered voters in each county were to cast ballots in the election — a conservative estimate in what’s expected to be a high-turnout election — the mail ballots could account for roughly half of the total vote in those counties.

There is a strong possibility that the immediate returns look more favorable to Trump, because in-person voting is expected to be heavily Republican. Democratic vote totals are likely to climb as the mail ballot counting goes on, a phenomenon known as “the blue shift.”

Some Democrats worry that if Trump leads even by a small amount on election night he will declare victory and try to short-circuit the vote count. If vast numbers of Democratic votes are reported from liberal strongholds like Philadelphia in the following days, the president has signaled he is ready to attack those results.

Lt. Gov. John Fetterman took to Twitter Thursday to praise counties that plan to begin counting mail ballots on Election Day and criticize those who are waiting until the next day.

“Let me be clear: Count every ballot in every county beginning on 7AM Election Day,” Fetterman wrote. “Every vote Equal.”

Street was less worried.

“I am concerned about getting an accurate count, not a quick count,” he said. “Each county has to make its own decisions when it comes to counting the votes. I think we should all settle in for the fact that Election Night we’re probably going to go to bed and there will be many votes yet to be counted.”

Staff writer Samantha Melamed contributed to this article