By Lawrence Hurley and Jan Wolfe
WASHINGTON (Reuters) - Siding with Wisconsin's Republican-led legislature, the conservative-majority U.S. Supreme Court refused on Monday to allow an extension ordered by a federal judge in the deadline for returning mail-in ballots in the state, dealing a setback to Democrats.
The court, in a 5-3 vote with three liberal justices dissenting, left in place a lower court's Oct. 8 decision that blocked U.S. District Judge William Conley's ruling that would have let officials count ballots that were postmarked by the time polls close on Election Day on Nov. 3 but arrived up to six days later.
The high court's order was issued just before the Senate voted to confirm President Donald Trump's nominee to fill a vacancy on the bench, Amy Coney Barrett.
The court action keeps in place a state policy that mail-in ballots be in the hands of election officials by the close of polls.
Wisconsin is crucial to Republican President Donald Trump's re-election chances against Democratic challenger Joe Biden.
Liberal Justice Elena Kagan wrote in a dissenting opinion that the court's decision "will disenfranchise large numbers of responsible voters in the midst of hazardous pandemic conditions."
The coronavirus pandemic is fueling an increase in voting by mail as Americans seek to avoid crowds at polling places, even as Trump makes repeated claims without evidence that such voting - long practiced in American elections - is rife with fraud. Elections experts have called such voter fraud exceedingly rare.
Trump's narrow victory in Wisconsin in 2016 helped him secure the presidency. A Reuters/Ipsos poll released on Monday showed Biden leading Trump by 53% to 44% in the state.
A group of Wisconsin voters and a disability rights groups, joined by state and national Democrats, sued the Republican-controlled state legislature to try to get the mail-in ballot receipt deadline extended in light of postal delays amid the pandemic.
Democrats argued that without an extension of the ballot-receipt deadline more than 100,000 voters in the state could be "disenfranchised through no fault of their own."
In a 2-1 ruling, a three-judge panel of the Chicago-based 7th U.S. Circuit Court of Appeals blocked Conley's earlier ruling allowing a six-day extension in the ballot-receipt deadline.
The 7th Circuit agreed with Wisconsin Republicans that it was too close to Election Day to make significant changes.
The Trump campaign issued a statement late on Monday welcoming the ruling and describing it as a "major victory" for Trump and the Republicans, and calling the demand for extension as the "Democrats' last-minute attempted rule changes".
Democrats in various Republican-governed states have decried what they call voter suppression efforts including opposition to measures intended to facilitate voting during the pandemic.
Other election cases are pending, which new appointee Barrett may cast crucial votes in.
Last week, the court split 4-4 in a case from Pennsylvania, handing a loss to Republicans hoping to curb the counting of mail-in ballots received after Election Day.