Many U.S. states expect high demand for mail-in absentee ballots in 2020 because voters are concerned about the risk of COVID transmission at crowded polling stations.
Election officials have security protocols to make sure the ballots are authentic, including special envelopes and signature checks.
Five U.S. states vote entirely by mail.
Nearly two-thirds allow citizens to request an absentee ballot for any reason.
Others require an excuse: active military duty, disability, travel.
Some worry voting by mail is subject to fraud and error.
Republicans have traditionally opposed all-mail elections, while Democrats have backed them.