(CNN) Mail-in ballots in Pennsylvania -- a critical state for President Donald Trump's reelection chances -- will be counted if they are received within three days of Election Day even if they do not have a legible postmark, the Supreme Court ruled Monday.
Four justices dissented from the order, signaling that the court was equally divided, with Chief Justice John Roberts siding with the court's three liberals.
The ruling is a loss for state Republicans who sought to require that only ballots received by Election Day be counted.
The highly anticipated order could set the tone for other pre-election challenges and highlights the fact that once again Roberts has moved left to side with his liberal colleagues in an area where he has a very conservative record. It also comes just two weeks before Election Day and intensifies the Supreme Court confirmation battle over Judge Amy Coney Barrett, who could well prove to be a deciding vote on election night challenges.
The Senate Judiciary Committee is set to vote Thursday to advance Barrett's nomination, with a full Senate vote expected early next week. That could diminish Roberts' influence on the court if Barrett sides with the other four conservative justices: Clarence Thomas, Samuel Alito, Neil Gorsuch and Brett Kavanaugh.
"Once again, Chief Justice Roberts played the same role he's played over the last year -- appearing to put what he views as the court's institutional interests ahead of what he otherwise might've preferred," said Steve Vladeck, a CNN legal analyst and professor at the University of Texas School of Law. "But with the confirmation of Judge Barrett seemingly a fait accompli, this may be one of the last times that such a move by him is decisive."
Pennsylvania Attorney General Josh Shapiro, a Democrat, told CNN's "New Day" on Tuesday that he's pleased with the ruling "because the voters here in Pennsylvania deserve clarity and confidence in an election that's already under way."
"Donald Trump has recklessly been trying to sow doubt in this process and limit the voices of the people of Pennsylvania. What this assures is that they'll count," he said.
The case in the battleground state has been of keen interest to voting rights experts who wanted to see how the Supreme Court might put limits on what state courts can do during the coronavirus pandemic to protect the right to vote. The justices have already signaled to federal courts that they should not step in to change rules too close to an election.
Earlier this month, Pennsylvania's state Supreme Court allowed officials to count mail-in ballots received up to three days after Election Day, even if the postmark was illegible.
The ruling would allow the votes of thousands of Pennsylvanians that could have been rejected for arriving after Election Day to be counted.
Lawyers for Republicans filed an emergency petition with the high court asking it to block the decision. They argued the state court opinion presents an "open invitation" to voters to cast their ballots after November 3, which would inject "chaos and the potential for gamesmanship into what was an orderly and secure schedule of clear, bright-line deadlines."
Lawyers for Democrats countered that the state's high court correctly acted after Pennsylvania's "recent experience of intractable voting problems in the primary election" during the pandemic.
This story has been updated with additional information about the court's action.