Before Judge Conley’s decision, state officials had estimated that at least 27,500 absentee ballots would come in too late to be counted — nearly five times the vote margin that decided the statewide judicial elections last year. As of Monday morning, more than 11,000 voters who requested ballots were never even sent one, according to data from the Wisconsin Elections Commission, though figures were continuing to update.

The number of disenfranchised voters was potentially higher. As of Monday, 185,000 absentee ballots remained outstanding, and election officials were trying to determine what percentage of those might have been returned had Judge Conley’s deadline of April 13 held. In Milwaukee, official tallies showed that the percentage of unreturned ballots was double its usual rate.

There was also the indeterminate number of voters who were too afraid to appear at polling stations on Election Day, by which point it would have been too late to request absentee ballots.

And then there was the matter of how to handle ballots that had not been postmarked with an exact date.

That question went at the heart of a seemingly arcane matter that is sure to bedevil states in November: whether to count mail-in ballots based on when they arrive or when they are postmarked, which, if Wisconsin is to be a guide, is already threatening to become the “hanging chad” of the 2020 election.

“Absentee ballots are going to be much more prevalent, and clerks around the country are going to be slammed the same way as they were in Wisconsin,” said Mr. Devaney, the lawyer who represented the Democrats in Wisconsin. How states settle on using postmarks, he added, “is going to be really important in terms of not disenfranchising thousands and thousands across the country.”