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 currently trails Democratic challenger Joe Biden.
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 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 also warned of a "rigged" election in 2016, as he trailed Hillary Clinton in the polls.
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 due to give a formal 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.
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.