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.
“The Dems talk of foreign influence in voting, but they know that Mail-In Voting is an easy way for foreign countries to enter the race,” he said.
Mr Trump also said postal voting was “already proving to be a catastrophic disaster” in areas where it was being tried out.
In June New York allowed voters to vote by post in the Democratic primary poll for the party’s presidential candidate. But there have been long delays in counting the ballots and the results are still unknown.
US media report that there are also concerns that many ballots will not be counted because they were not filled in correctly or do not have postmarks on them that show they were sent before voting officially ended.
However, several other states have long conducted votes by post.