Election officials in Wisconsin are redoubling efforts to persuade voters to return their mail ballots as soon as possible after the Supreme Court ruled Monday night that ballots received after Election Day cannot be counted, no matter when they were mailed.

As of Tuesday, voters in the key battleground state had returned more than 1.45 million of the 1.79 million absentee ballots they had requested so far — a return rate of more than 80 percent.

But that means that nearly 327,000 absentee ballots had not yet been returned. And voters continue to request ballots — under state law, they have until 5 p.m. Thursday to seek one, a deadline state officials have warned is probably too late for voters to receive and return a ballot by mail before Election Day.

Even before the Supreme Court decision, state officials had warned that Tuesday was the “practical deadline” for voters to mail their absentee ballots and ensure they arrive by 8 p.m. on Election Day, as now required by the court ruling.

Starting Wednesday, they urge anyone who had not already mailed their ballot to instead deliver it in person to a clerk’s office, place it in an official drop box or vote in person instead.

“We want citizens to choose the option for voting that works best for them, but time is running out,” Meagan Wolfe, administrator for the Wisconsin Elections Commission, said in a statement issued after Monday’s decision.

Wisconsin state law prohibits counting ballots that arrive after polls close on Election Day, even if voters mailed them before the deadline. Democrats and civil rights groups had sought to extend by six days the deadline of when ballots needed to be received, arguing that a surge in absentee voting amid the coronavirus pandemic and a slowdown in mail delivery meant large numbers of voters could otherwise be disenfranchised.

During the state’s primary elections in April, courts approved a similar deadline extension, resulting in the counting of more than 79,000 late-arriving ballots that were postmarked by Election Day, according to the state elections commission. In the spring contests, more than 10 percent of the vote in the Democratic strongholds of Madison and Milwaukee arrived after Election Day.

A federal court had initially agreed to extend the ballot deadline for the November election, as well. But in a 5-to-3 decision, the Supreme Court sided with the Republican National Committee, the Wisconsin Republican Party and the state’s GOP-led legislature in deciding to maintain the Election Day deadline.

The court, which now has a full complement of justices with the confirmation Monday of Amy Coney Barrett, is likely to consider similar issues in other key states in coming days. In North Carolina, Republicans are also challenging a deadline extension for ballots. And Pennsylvania Republicans are now asking the court to reconsider its 4-to-4 tie last week that allowed a deadline extension of three days for ballots in that state to stand.

In Wisconsin, late ballots could be critical if the election is close. In 2016, Donald Trump defeated Democratic nominee Hillary Clinton by fewer than 23,000 votes in the state.

Andrew Hitt, chairman of the Wisconsin Republican Party, praised the Supreme Court’s ruling, saying it would help dispel voter confusion by preventing a last-minute deadline change.

“We have plenty of time. Everybody knows what the rules are,” he said.

Hitt said that if the vote is close in Wisconsin, the decision would help prevent a “Florida 2000” situation, in which the parties fight tooth and nail over individual ballots that arrive after Election Day, some with smudged or missing postmarks.

Democrats decried the ruling, saying it could prevent voters from participating in the election. In a tweet, state party chair Ben Wikler wrote that the court had “failed an opportunity to expand democracy.”

But some Democrats were cautiously optimistic that months of voter education about the need to return ballots promptly might blunt the impact of the ruling.

In April, changing and confusing rules issued amid the still-new pandemic contributed to an especially high number of absentee ballots being cast at the last minute, officials said.

But in the run-up to the Nov. 3 general election, there have been historic rates of early voting, both by mail and in person.

State tallies show hundreds of thousands of Wisconsin voters requested their absentee ballots early and returned them promptly. Ballot requests have been slowing in recent days.

“We never counted on this [extension] being in place,” said Reid Magney, spokesman for the Wisconsin Elections Commission. “Our messaging all summer has been, ‘Make your request early, then return it as soon as possible.’ ”

Meanwhile, polls have showed that former vice president Joe Biden has had a consistent lead in the state, to some extent lessening the likelihood that late-arriving ballots will make the difference in the presidential race. A Washington Post average of polls since Oct. 16 shows Biden ahead by eight percentage points in Wisconsin.

“The Wisconsin voter is much more sophisticated and educated this election cycle,” said Michael Maistelman, a Democratic elections lawyer in Wisconsin who has represented Gov. Tony Evers (D) and former U.S. senator Russ Feingold (D), among others. “We have also done an amazing job at all levels of education the electorate.”

Still, he said, state voters have come “to understand that voting rights in Wisconsin is like riding a roller coaster,” the rules buffeted by changing decisions by state and federal courts.

Magney said he expected some proportion of people who requested an absentee ballot will instead choose to vote in person. Some voters made their requests for mail ballots in the spring, and since that time, he noted, concerns about mail delivery have grown,

Data released by the U.S. Postal Service showed that first-class mail in the region that includes Wisconsin was arriving on time less than 85 percent of the time in mid-October, worse than the national average and far below the Postal Service’s targets. There are some indications that the Postal Service is doing better, however, at delivering ballots, which are supposed to receive priority treatment.

Wikler tweeted Monday night that Democrats were “dialing up a huge voter education campaign” to reinforce the deadline and ensure no late-arriving ballots.

“We’re phone banking. We’re text banking. We’re friend banking. We’re drawing chalk murals, driving sound trucks through neighborhoods, & flying banners over Milwaukee,” he wrote. “We’re running ads in every conceivable medium.”

Jacob Bogage and Emily Guskin contributed to this report.