Democrats in the state have asked the court to allow the counting of ballots to arrive up to six days after Election Day if postmarked by November 3.
The ruling came before the Senate voted 5-3 to include Judge Amy Connie Barrett in the Supreme Court.
Justice Elena Kagan, along with her generous colleagues, Justice Sonia Sotomayor and Stephen Breyer, opposed the court order.
Last week, a Pennsylvania state court upheld a ruling that extended the mail-in ballot deadline.
The difference between the Pennsylvania and Wisconsin cases
Unlike the Pennsylvania order last week, the Wisconsin order is related to a ruling from a lower federal court on Monday, not a state court, and Chief Justice John Roberts made the distinction.
A federal district court in Wisconsin sided with Democrats to receive mail-in ballots for up to six days after Election Day, but the Court of Appeals overturned the order and the Supreme Court upheld the block.
The Federal District Court wrote an opinion that Roberts “interfered during the election season” to block a state law. He said the case referred to “federal intrusion on the processes of the state legislatures.”
On the other hand, the Pennsylvania case is related to a decision given by the state Supreme Court. The decision “represents the power of state courts to apply their own constitutions to electoral regulations,” Roberts said.
“Different laws and different ancestry govern these two conditions, and in these special circumstances, we need to allow the amendment of electoral rules in Pennsylvania, but not Wisconsin,” Roberts wrote.
Steve Vladek, a CNN legal analyst and professor at the University of Texas School of Law, said it was unclear how long the court would make that difference since Barrett became the ninth member.
“The only justice that sees these cases differently than those from Pennsylvania is the Chief Justice, for whom there is a taxonomic difference between extending a state Supreme Court election deadline, refusing to extend it, and the federal district court doing the same,” Vladek said. “But the central role of the chief justice may be short-lived, with Justice Barrett able to vote on the next election cases.
During the epidemic, getting ballots on time
In her dissent, Kagan, Brier and Sotomayor joined in, blaming the majority for abandoning voter rights during the epidemic.
“Outraged by the COVID pandemic, the court failed to adequately protect Nation voters,” Kagan wrote.
Voting groups in Wisconsin have emphasized the importance of submitting ballots in a timely manner.
The Wisconsin Election Commission said in a statement on Monday that it was “constantly urging voters to request and return their absentee ballots as soon as possible to ensure they receive them within the legal deadline of 8pm on election day. Tonight’s verdict will do nothing to change that.”
This article has been updated with additional information from the judgment and comment.
CNN’s Caroline Kelly, Jeremy Herb, Casey Tolan, Carolyn Kenny and Ellie Kaufman contributed to the report.