The decision means that the court would not consider the decision until after Election day, even though the court's three most conservative members declared they would prefer to decide even sooner.
Justice Amy Coney Barrett, who took the bench for the first time Tuesday, did not take part in considering the case.
US Democrats Lead Vote-By-Mail Registers in Florida
Conservative justice Samuel Alito joined two of his colleagues in stating: “I reluctantly conclude that there is simply not enough time at this late date to decide the question before the election.”
While the GOP's request failed to garner the vote of five justices necessary to expedite a review of the case, Wednesday's denial does not limit the court from taking up the pending GOP request for a ruling based on its merits, which would only require approval from four of the court's nine justices.
Justices Clarence Thomas and Neil Gorsuch joined Alito in saying that the court could still act in time to impact how the votes are counted in Pennsylvania, arguing that the state's court-ordered extension is "likely" unconstitutional.
Pennsylvania Attorney General Josh Shapiro (D) applauded the court for not fast-tracking the case, while acknowledging that the court could still act on the GOP request: "The denial of expedited review is good for Pennsylvania voters, who will not have the rules changed on them on the eve of the election without proper review,” he said. “We know this fight may not be over and we are prepared.”
Pennsylvania is a key battleground state of nearly 13 million people, which President Trump won in 2016 by just 45,000 votes. Studies show that suporters of Democratic candidate Joe Biden are twice as likely as Trump voters to vote by mail, and the Supreme Court decision comes amidst Trump and his Republican allies' claims that easing voting rules opens elections up to widespread fraud.
Wednesday's decision comes in response to the Pennsylvania GOP's second recent attempt to roll back the state's mail-in ballot extension. Last week the Supreme Court was deadlocked 4-4 on the extension, thus leaving it intact, with Chief Justice John Roberts siding with the three liberal justices in denying the GOP's request to halt the state court ruling.