US Postal Service controversy: why Donald Trump blocked funds to the USPS - and if new chief Louis DeJoy is trying to fix the election
This form of voting allows citizens to cast their votes in the presidential election, while not having to attend their local polling station and risk contracting the virus.
The worrying news comes as experts revealed concern about catching Covid-19 could keep up to half of the electorate away from in-person voting.
Here's everything you need to know.
Why is Donald Trump against postal votes?
As opinion polls continue to show Trump trailing in popularity to presumed Democratic nominee Joe Biden, the President obviously sees mail-in ballots as a threat to his position.
His opinions on the system were first made known in May when he tweeted, "There is NO WAY (ZERO!) that Mail-In Ballots will be anything less than substantially fraudulent.
"Mail boxes will be robbed, ballots will be forged & even illegally printed out & fraudulently signed."
His claims were deemed to be so unsubstantiated, Twitter added a fact-check label to the President's posts for the first time in history, and said Trump "falsely claimed that mail-in ballots would lead to 'a Rigged Election’.
"However, fact-checkers say there is no evidence that mail-in ballots are linked to voter fraud.”
Are postal votes secure?
Five states, including Washington, Oregon and Colorado, currently conduct their elections entirely via mail-in ballot, and experts have said the mail-in voting system is safe from tampering. It is a system that Trump himself has used in the past.
Trump claimed that California will send mail-in ballots to "anyone living in the state, no matter who they are or how they got there."
In fact, only registered voters will receive ballots.
What's happening now?
Ahead of the election, the United States Postal Service (USPS) has warned that millions of mail-in ballots may not arrive in time to be counted in the election. Critics have blamed Louis DeJoy, the new head of the USPS, and an outspoken and loyal Trump supporter.
In a letter published on Sunday 16 August, Nancy Pelosi, the Speaker of the US House of Representatives, described DeJoy as "one of the top Trump mega-donors", and said he has "proven a complicit crony as he continues to push forward sweeping new operational changes."
According to Pelosi, these changes "degrade postal service, delay the mail, and - according to the Postal Service itself - threaten to deny the ability of eligible Americans to cast their votes through the mail in the upcoming elections in a timely fashion.”
Previously, President Trump told Fox News he would be denying additional funding to the USPS, saying "if we don't make a deal, that means they don't get the money. That means they can't have universal mail-in voting, they just can't have it."
What can be done?
Pelosi is leading the charge to protect America's Postal Service.
She's recalling the House - which was expected to be in recess until September unless a coronavirus relief agreement required a vote - and urging representatives to vote on a new bill that would prohibit the USPS from introducing any changes to its services.
The Delivering for America Act would not allow the Postal Service to implement any changes to operations or level of service it had in place on 1 January 2020.
A date for the vote has not yet been announced, but it is expected to take place in the coming week.