Donald Trump has called for November's presidential election to be postponed, saying increased postal voting could lead to fraud and inaccurate results.
He suggested a delay until people can "properly, securely and safely" vote.
There is little evidence to support Mr Trump's claims but he has long railed against mail-in voting which he has said would be susceptible to fraud.
US states want to make postal voting easier due to public health concerns over the coronavirus pandemic.
In a series of tweets, Mr Trump said "universal mail-in voting" would make November's vote the "most inaccurate and fraudulent election in history" and a "great embarrassment to the USA".
He suggested - without providing evidence - that mail-in voting, as it is known in the US, would be susceptible to foreign interference.
Mr Trump also said postal voting was "already proving to be a catastrophic disaster" in areas where it was being tried out.
Earlier this month, six US states were planning to hold "all-mail" ballot elections this November: California, Utah, Hawaii, Colorado, Oregon and Washington.
These states will automatically send postal ballots to all registered voters, which then have to be sent back or dropped off on election day - although some in-person voting is still available in certain limited circumstances.
About half of US states allow any registered voter to vote by post on request.
Critics of postal voting argue that people could vote more than once via absentee ballots and in person. Mr Trump has in the past said there was a risk of "thousands and thousands of people sitting in somebody's living room, signing ballots all over the place".
However, there is no evidence of widespread fraud, according to numerous nationwide and state-level studies over the years.