The head of the United States Postal Service (USPS) is due to give evidence before senators on Friday following allegations of attempts to sabotage this year's presidential election.

US postmaster general Louis DeJoy, a key ally of President Donald Trump, took control of the postal service in June and swiftly engineered cuts and operational changes that have disrupted mail delivery operations.

Critics fear widespread delays could disrupt November's election, which is expected to bring a surge of postal votes - known as mail-in ballots in the US because of the coronavirus pandemic.

According to a recent survey by the Democracy Fund & UCLA Nationscape project, 37% of registered voters are planning to cast their ballot through the post, ensuring a socially distant process.

The same survey found more Democrat voters planned to vote by mail than Republican - 48% versus 23%.

At the end of July, it wrote to 46 states warning it can't guarantee that all mailed-in ballots will arrive in time to be counted, which could render them invalid and disenfranchise a large chunk of the electorate.

They are, largely, the same. An absentee ballot is posted by someone absent from their local voting jurisdiction who requests a vote through the mail.

Mail-in ballots are when a state pro-actively sends out mail ballots to voters, regardless of whether they request it or not.

President Trump endorses the absentee ballot - in fact, he uses one to vote in Florida.

He views mail-in ballots rather differently, warning repeatedly that it's prone to voter fraud and could corrupt the election - but there is little evidence to support this.

States have rigorous procedures to ensure the security of ballot papers.

Recent analysis by The Washington Post of three states that conduct universal mail elections found a "miniscule" rate of ballots that were potentially fraudulent.

The Democrats are demanding Mail-In Ballots because the enthusiasm meter for Slow Joe Biden is the lowest in recorded history, and they are concerned that very few people will turn out to vote. Instead, they will search & find people, then “harvest” & return Ballots. Not fair!

— Donald J. Trump (@realDonaldTrump) August 20, 2020

Donald Trump claims (without evidence) that mail-in voting will create a free-for-all on cheating, forgery and theft which will lead to a rigged election.

He has speculated that Americans will "maybe never know" the election result and floated the idea of delaying the election and staging a rerun.