Thousands of absentee ballots normally get rejected each presidential election. But with an increase in mail-in voting this year, it could be worse in 2020.
If ballots are turned down at the same rate as this year's primaries, that could mean a significant number of voters might not get a say.
An analysis by The Associated Press says up to three times as many ballots in key swing states could get rejected, compared to the 2016 presidential election, reflecting the increased number of votes being cast by mail.
Ballots can go uncounted for arriving too late, if voters forget to sign them or if signatures don't match what's on file at election offices.
The AP analysis say urban areas could be affected the worst, damaging Democratic candidates. Studies show new, young, Black and Hispanic voters are disproportionately affected by rejected ballots.
The U.S. Postal Service is recommending voters allow seven days for their ballot to reach an election office.