US President Donald Trump has called for November's presidential election to be postponed, saying increased postal voting could lead to fraud and inaccurate results.
Trump suggested a delay until people can "properly, securely and safely" vote, reports the BBC.
There is little evidence to support 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 tweet, 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".
Donald Trump suggested that mail-in voting 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.
He also said postal voting was "already proving to be a catastrophic disaster" in areas where it was being tried out.
Six US states - California, Utah, Hawaii, Colorado, Oregon and Washington, were planning to hold "all-mail" ballot elections in November. The 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.
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".