President Donald Trump adopted a grim tone in remarks to Republicans who formally backed his bid for a second term on Monday local time, warning without evidence that he could face a “rigged election” in November.

Mr Trump repeated his claim that voting by mail, a longstanding feature of American elections that is expected to be far more common during the coronavirus pandemic, could lead to an increase in fraud.

Independent election security experts say voter fraud is quite rare in the United States.

Mr Trump spoke in an unscheduled appearance on the first day of the sharply scaled-back Republican National Convention in Charlotte, North Carolina, after he received enough votes to formally win the nomination to take on his Democratic rival, former Vice President Joe Biden, in the November 3 election.

“The only way they can take this election away from us is if this is a rigged election,” Mr Trump said.

“We’re going to win this election.”

Tweet from @GOPconvention

Party members are meeting amid a pandemic that has killed more than 176,000 Americans, erased millions of jobs and eroded the president’s standing among voters.

As he has done repeatedly, Mr Trump described states’ responses to infections of COVID-19, the illness caused by the coronavirus, in starkly partisan terms, casting lockdowns and other steps recommended by public health officials as attempts to influence voting in November.

“What they’re doing is using COVID to steal an election,” Mr Trump said.

“They’re using COVID to defraud the American people – all of our people – of a fair and free election.”

Former US Vice President Joe Biden and wife Jill wear face masks to watch a fireworks display during the closing moments of the virtual 2020 Democratic National Convention, livestreamed online. Photo: Getty

The in-person proceedings, a far smaller meeting than originally planned, still marked a contrast with Democrats, who opted for an almost entirely virtual format instead of gathering in the election battleground state of Wisconsin.

That change was intended to reduce the risk of the virus being spread at the political event.

“We did this out of respect for your state,” said Mr Trump, targeting his message at the people of North Carolina, which is expected to be competitive in the November election.

Mr Trump earlier this year moved the convention to Florida, his newly-adopted home state, to avoid restrictions on gatherings in North Carolina due to the coronavirus, then abandoned that plan when infection rates soared in Florida.

Mr Biden, 77, is leading Mr Trump, 74, in opinion polls.

Mr Biden and his fellow Democrats portrayed Mr Trump as a force for darkness, chaos and incompetence during their convention, while stressing the Democrats’ diversity and values like “empathy” and “unity.”

Republicans said their convention would offer a more hopeful message, with an emphasis on “law and order,” gun rights, tax cuts and the “forgotten” men and women of America.

The party opted not to vote on a traditional platform document detailing its policy goals, instead saying that it supports what Mr Trump is doing.

Mr Trump’s campaign released a series of bullet-point goals, including a promise to “create 10 million new jobs in 10 months.”

In another contrast with the Democratic event, which featured all three living former Democratic presidents, and prior nominees, the Republican event will not include speeches from that party’s past living president or candidates.

Neither former President George W Bush nor 2012 Republican presidential nominee Senator Mitt Romney, who voted to convict Mr Trump at the president’s impeachment trial, plan to speak.

Also absent from the schedule are several Republicans facing close elections in November, including Senator Thom Tillis of North Carolina.

-AAP