Many states allow local election clerks to verify mail-in ballots days or even weeks ahead of Election Day.
That gives them a head start when tabulating the results on all ballots begins.
But in some of the key presidential battleground states, clerks are prohibited from doing that or given very limited time to do so, slowing the count for what is expected to be a crush of mail-in ballots this year.
Many worry that any delay in results could give President Donald Trump more room to continue his unfounded attacks on the electoral system.