The Supreme Court refused to extend Wisconsin's deadline to receive and count ballots from absentee voters.
On Monday, justices ruled 5-3 along partisan lines that mail-in ballots will only be counted if received by Election Day on Nov. 3.
The following day, Democrats and Republicans in the battleground state were pushing to get 320,000 outstanding absentee ballots returned by the close of polls on Election Day.
Democrats previously argued in a federal lawsuit that more time should be allotted for ballots to arrive by mail and be counted because of challenges posed by the coronavirus pandemic. Meanwhile, Republicans countered that voters had plenty of options to vote on time and that the rules shouldn't be changed so close to the election.
The Supreme Court sided with Republicans to block an extension of a state law to count votes until Nov. 9 as long as they were postmarked by Nov. 3.
Last week, the high court allowed a mail-in voting deadline extension in Pennsylvania.
Justice John Roberts explained his switch, saying the Pennsylvania case involved state court rulings while in Wisconsin, a federal court had allowed the extension.
Additional reporting by Scott Bauer of The Associated Press.