A poll by Pew Research Center found that 52% of Americans said voter fraud was a major or minor problem of vote-by-mail in presidential elections.
Many Republicans, including President Trump, have argued that stricter voter registration measures, such as ID checks, are necessary to prevent voter fraud. Trump claimed that mail-in ballots had been found in waste paper baskets and in rivers.
Other fears have circulated about foreign states printing counterfeit ballots to interfere in the election outcome, which many see as legitimate given investigations into Russian interference in the 2016 election.
Concerns have also been raised that voters may vote-by-mail and again in person on election day without further registration checks - voting twice is illegal in all 50 states.
The Heritage Election Fraud Database details nearly 1,300 proven cases of election fraud, including individuals stealing mail-in-ballots from post boxes.
Whether these fears are well-grounded or not, the attention they have received has led to questions about the legitimacy of the election, and legal battles after election day may challenge the outcome. Faith in the democratic process is crucial to its operation, and the threat of voter fraud, whether real or imagined, shows US democracy is at breaking point.