A shift to mail voting is increasing the chances that Americans will not know the winner of November’s presidential race on election night, a scenario that is fueling worries about whether President Donald Trump will use the delay to sow doubts about the results.

State election officials in some key battleground states have recently warned that it may take days to count what they expect will be a surge of ballots sent by mail out of concern for safety amid the pandemic. In an election as close as 2016’s, a delayed tally in key states could keep news organizations from calling a winner.

“It may be several days before we know the outcome of the election,” Jocelyn Benson, Michigan’s Democratic secretary of state, said in an interview. “We have to prepare for that now and accept that reality.”

Ohio’s Republican secretary of state, Frank LaRose, pleaded for “patience” from the public. “We’ve gotten accustomed to this idea that by the middle of the evening of election night, we’re going to know all the results,” LaRose said Wednesday at a forum on voting hosted by the Bipartisan Policy Center. “Election night reporting may take a little longer” this year, he warned.

Delayed results are common in a few states where elections are already conducted largely by mail. But a presidential election hasn’t been been left in limbo since 2000, when ballot irregularities in Florida led to weeks of chaos and court fights.

For some election experts and Democrats, the prospect of similar uncertainty is especially worrisome this year, as Trump disparages mail-in voting as fraudulent and has claimed without evidence that widespread mail balloting will lead to a “rigged” election.