US President Donald Trump has warned his fellow Republicans their opponents may “steal” November’s election, as his party anointed him as their candidate.
“They’re using Covid to defraud the American people,” Mr Trump told delegates on the first day of the party convention in North Carolina.
Mr Trump repeated his much-disputed claims that mail-in ballots could lead to voter fraud.
Opinion polls suggest he is lagging behind Democratic challenger Joe Biden.
Mr Biden, the former vice-president to Barack Obama, has boasted a 10-point lead on occasions.
Addressing delegates in person at a party conference that has been dramatically scaled back by Covid-19, Mr Trump accused Democrats of “using Covid to steal an election”.
The US president has repeatedly suggested an unprecedented surge in postal voting could lead to widespread fraud even though experts and voting officials say this is a false conspiracy theory.
“The only way they can take this election away from us is if this is a rigged election,” he said. “We’re going to win.”
Mr Trump had also warned of a “rigged” election in 2016, as he trailed Hillary Clinton in the polls.
But in 2016 the polls were far less clear and just a few percentage points separated Mr Trump and his then-rival Hillary Clinton at several points as election day neared.
On Monday, Mr Trump was officially nominated as a formality to be the Republican nominee at his party’s convention in the city of Charlotte.
Supporters cheered him, chanting: “Four more years!”
The president is expected to make live television addresses on every day of the convention, leading up to his acceptance speech to the party jamboree on Thursday. It is unusual for candidates to address the convention before that point, as Mr Trump has done.
Are mail-in ballots safe?
Mr Trump has repeatedly asserted that expanded mail-in voting – which is expected to happen at an unprecedented level due the coronavirus pandemic – will lead to “the most corrupt election” in US history.
But there is scant evidence of widespread voter fraud, and very few examples of any related criminal prosecutions. The rate of voting fraud overall in the US is between 0.00004% and 0.0009%, according to a 2017 study by the Brennan Center for Justice.
Ellen Weintraub, commissioner of the Federal Election Commission, has said: “There’s simply no basis for the conspiracy theory that voting by mail causes fraud. None.”
Mail-in voting is used by the US military, and even Mr Trump himself and members of his family.
But a recent slowdown in mail deliveries due to cost-saving measures in the US postal system has fuelled concerns that ballots might not be returned by election day.
Several states have sought to change their election laws to allow ballots to be counted days after the forthcoming presidential vote, which some analysts fear could lead to delays in declaring the presidential victor.
A primary election in New York City this June took weeks to determine a winner after poll officials were deluged with 10 times the normal number of mail-in ballots. There was no allegation of fraud, but the debacle raised fears of a protracted vote count this November.
Earlier this month, a New Jersey judge ordered a new vote after finding evidence of fraud in a May election that was conducted entirely by mail in Paterson. Four people were arrested, including a local city councilman and councilman-elect. The case has been frequently touted by the Trump campaign.