BULLHEAD CITY/WILMINGTON (REUTERS) - With less than a week before Election Day, President Donald Trump held an in-person campaign rally in Arizona on Wednesday (Oct 28) despite a US surge in Covid-19 cases and criticism he is prioritising his re-election above the health of his supporters.

The pandemic that has upended life across the United States this year, killing more than 227,000 people, is roaring back in the days leading up to Tuesday's contest between Republican Trump and Democratic challenger Joe Biden.

Mr Biden holds a comfortable lead in national polls, which show a public increasingly dismayed by Mr Trump's handling of the largest public health crisis in US living memory. Polls in battleground states that will likely decide the election are tighter than the national surveys.

At a campaign rally in Bullhead City, Arizona, Mr Trump said again that a vaccine would be available soon.

"If I weren't president, if you had Sleepy Joe as your president, it would have taken you four years to have a vaccine. You would have never had a vaccine," Mr Trump said.

A number of drugmakers are competing to bring a coronavirus vaccine to market, but one is not expected to be ready before next week's election.

A Trump adviser said the rallies were "priceless" given Mr Biden's dominance on the ad airwaves.

"These rallies generate significant free media coverage that helps offset whatever money advantage Biden has. Fox News basically runs the whole rallies. It's a great asset," the adviser said.

Mr Biden raised about US$130 million (S$177 million) during the Oct 1-14 period, about three times the roughly US$44 million raised by Mr Trump's campaign, according to disclosures filed last Thursday with the Federal Election Commission.

The adviser said Mr Trump was typically holding rallies in portions of the country where masks are worn less and people are less concerned about the ill effects of Covid-19.

"If you overlay the rallies on a map, you would see they typically run along the same lines of the divide over Covid. So, really, there's only upside," the adviser said.

But Mr Kyle Kondik, a political analyst at the University of Virginia's Centre for Politics, said the rallies could backfire.

"More exposure doesn't necessarily lead to more votes, and I wonder if some on the fence might be turned off by the total disregard for Covid protocols," Mr Kondik said.

A pro-Biden group, Priorities USA Action, said its recent polling in six battleground states found that when people were told Mr Trump was holding large rallies without mask-wearing requirements, most disapproved.

After a briefing from public health officials on Wednesday, Mr Biden slammed what he called the Trump administration's disregard for safety and failure to develop a plan to contain Covid-19. Polls show Americans trust Mr Biden more than Mr Trump to contain the virus.

"The longer he's in charge the more reckless he gets," Mr Biden told reporters, before casting his own vote in Wilmington, Delaware.

Mr Trump will hold a rally in Goodyear, outside Phoenix, Arizona's biggest city, later on Wednesday.

In all, Mr Trump plans to visit 10 states in the last week of the campaign and will host 11 rallies in the final 48 hours, a campaign official said.

New battleground state polls from Reuters/Ipsos show Mr Biden and Mr Trump in a statistical dead heat in Arizona and in Florida, another key battleground state.

More than 75 million people have cast early in-person and mail ballots, according to data compiled by the US Elections Project at the University of Florida. That is a record-setting pace and more than 53 per cent of the total 2016 turnout.

'Imminent risk'

Mr Trump's running mate, Vice President Mike Pence, campaigned on Wednesday in Wisconsin, a state that could help decide the election.

Wisconsin broke one-day state records in cases and deaths on Tuesday. State officials have asked residents to quarantine voluntarily, wear masks and cancel social gatherings with more than five people. Each of Mr Trump's rallies in recent days has drawn several thousand people.

"There's no way to sugarcoat it - we are facing an urgent crisis and there is an imminent risk to you, your family members, your friends, your neighbours, and the people you care about," Democratic Governor Tony Evers said. "Stay home, Wisconsin."

The Trump campaign was taking all necessary precautions at Mr Pence's event, campaign spokesman Hogan Gidley told CNN.

Asked whether holding a big rally raised safety concerns given that aides to Mr Pence have tested positive and Wisconsin hospitals are near capacity, Mr Gidley said: "No, it doesn't...The vice president has the best doctors in the world around him. They've obviously contact traced and come to the conclusion it's fine for him to be out on the campaign trail."

Reuters/Ipsos polling from Oct 20 to 26 shows Mr Biden with a solid lead over Mr Trump, 53 per cent to 44 per cent, in Wisconsin, with 52 per cent of voters saying Mr Biden would be better suited to handle the pandemic and 38 per cent favouring Mr Trump on the issue.

US stocks tumbled on Wednesday, with the Dow closing at lows last seen in late July, as coronavirus cases soared globally and investors worried about the possibility of a contested US presidential election next week.

Epidemiologists say rallies posed grave public health risks during the pandemic because people shout and chant while packed closely together, aiding to the spread of the virus.

"Rallies are perfect places for so-called super-spreader events," said Dr Barun Mathema, an epidemiologist at Columbia University in New York, adding that the risk increases when attendees do not wear masks, which is the case for many of the people attending Trump events.

Officials in Michigan and Minnesota told Reuters they identified people who attended Trump campaign events in their states and later tested positive for Covid-19, although it was not clear if they were first exposed to the virus at the rallies or elsewhere.