WASHINGTON, Aug. 24 (Reuters) - President Donald Trump adopted a grim tone in remarks to Republicans who formally backed his bid for a second term on Monday (Aug 24), warning without evidence that he could face a "rigged election" in November.

Trump repeated his assertion that voting by mail, a long-standing 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.

Trump spoke 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 Nov 3 election.

"The only way they can take this election away from us is if this is a rigged election," Trump said. "We're going to win."

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, 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," Trump said. "They're using COVID to defraud the American people - all of our people - of a fair and free election."

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.

Biden, 77, is leading Trump, 74, in opinion polls. Biden and his fellow Democrats portrayed Trump as a force for darkness, chaos and incompetence during their convention, while stressing the Democrats' diversity and values like "empathy" and "unity".