If voters who requested absentee ballots change their minds and try to vote on Election Day, they may run afoul of a thicket of rules that vary from state to state. In many states, switching from absentee to in-person requires a series of steps to cancel the absentee ballot. Voters may be asked to cast provisional ballots that take longer to process. All these last-minute changes take more resources and more time and introduce the possibility for errors.

“It puts a whole lot of pressure on already overburdened election officials,” said Wendy Weiser of the Brennan Center for Justice.

Still, some Democratic groups trying to turn out voters have shifted their messaging from voting by mail to voting in person. In Philadelphia, the result is an unhelpful “whiplash,” said Al Schmidt, an elections commissioner in the city who worries about the unexpected impact.

“It adds to lines and has more people in polling places longer, which is what we’re trying to avoid,” he said.