In a 5-3 ruling, the justices refused to reinstate a lower court order that called for mailed ballots to be counted if they are received up to six days after the Nov. 3 election. A federal appeals court had already put that order on hold.
Democrats argued that the flood of absentee ballots and other challenges posed by the coronavirus pandemic makes it necessary to extend the period in which ballots can be counted. Wisconsin is one of the nation's hot spots for COVID-19, with hospitals treating a record high number of patients with the disease.
Republicans had opposed extending the Nov. 3 deadline, saying that voters have plenty of opportunities to cast their ballots by the close of polls on Election Day and that the rules should not be changed so close to the election.
The Wisconsin Department of Health Services said that more than 201,000 people in the state have tested positive for COVID-19 as of Monday.
The 7th Circuit Court of Appeals had approved a six-day extension of a deadline for absentee ballots to be received in Wisconsin. Absentee ballots postmarked on or before Election Day would have been counted as long as they came in before Nov. 9.
Chief Justice John Roberts sided with his conservative colleagues. Justice Elena Kagan, Stephen Breyer, and Sonia Sotomayor dissented.
Fox News’ Tyler Olson and The Associated Press contributed to this report.