President Donald Trump has urged states to shun lockdowns as his Democratic rival Joe Biden said the pandemic could not be stopped by "flipping a switch".
Continuing a whirlwind schedule of rallies in battleground states, Mr Trump also mocked mask mandates.
Mr Biden said Mr Trump's handling of America's worsening coronavirus crisis was an "insult" to its victims.
The Democrat has a solid national lead over the Republican president six days before the 3 November election.
But Mr Biden's advantage is narrower in the handful of US states that could vote either way and ultimately decide who wins the White House.
More than 75 million Americans have voted early, nearly 50 million of them by post, in a record-breaking voting surge driven by the pandemic.
Virus deaths are rising in 39 US states and an average of about 800 people are dying daily nationwide.
White House coronavirus task force expert Dr Anthony Fauci told the BBC on Wednesday that Mr Trump's political rallies were bound to spread Covid-19. He said gatherings of people not wearing masks or socially distancing were potentially superspreading events.
Infection rates are also rising sharply across Europe, including in the UK, France and Germany.
Global stock markets plunged on Wednesday amid fears that the pandemic could reverse tepid economic recoveries.
Biden pledges to 'do right things' in Covid fight
The two presidential rivals' divisions over the coronavirus were on stark display once again on Wednesday.
Speaking from his home of Wilmington, Delaware, Democrat Joe Biden said he would not campaign "on the false promises of being able to end this pandemic by flipping a switch".
Mr Biden - who has not ruled out further lockdowns - pledged instead to "let science drive our decisions".
"Even if I win, it's going to take a lot of hard work to end this pandemic," he said. "I do promise this: We will start on day one doing the right things."
He also railed against Mr Trump's attempts to dismantle the Affordable Care Act, also known as Obamacare, in the midst of a pandemic.
The Democrat and his wife Jill Biden cast their early ballots soon afterwards. Mr Trump voted early on Saturday in Florida.
Trump: Biden would lockdown America
At a rally in Goodyear, Arizona, Mr Trump warned that a Biden presidency would lead to more lockdowns and economic misery for Americans.
"If you vote for Joe Biden it means no kids in school, no graduations, no weddings, no thanksgivings, no Christmas, and no Fourth of July together.
"Other than that you'll have a wonderful life. Can't see anybody, but that's alright."
He cast the election as "a choice between a Trump super-recovery and a Biden depression".
At an earlier rally in Bullhead City, also in Arizona, a state where looser rules on social distancing make staging crowded events easier, Mr Trump poked fun at mask mandates in Democratic-run states.
An F-16 fighter jet was scrambled during the rally to ward off an aircraft that entered the air space without authorisation. The warplane fired flares to get the pilot's attention and the smaller aircraft was escorted out of the area without further incident, according to the North American Aerospace Command.
The mid-air interception caught the notice of the president, who looked up at the roaring jet and said: "I love that sound."
An emotional punch
At every rally stop on this multi-state, cross-country campaign swing, Donald Trump - in his words and in his actions - is trying to convince the country that the coronavirus pandemic is on the verge of being "vanquished".
At times, he warns that the cure - aggressive mitigation efforts - cannot be worse than the disease. At others, he downplays the severity of the pandemic.
"I caught it," he told the crowd at one of his Arizona rallies. "And then you get better. And then you get immune."
Sometimes he boasts about the effectiveness of his response and how, if he hadn't acted, two million Americans would have died (this is a number experts say would have been approached only if the government had done nothing at all).
Donald Trump is reaching for a way to address a cold truth - that opinion polls indicate the public believes the coronavirus pandemic is the most pressing issue facing the nation, and the president is not doing a good enough job in handling it.
With only five days before the election, and with 75 million Americans having voted so far, time is running out for the president to change minds. But in a relentless string of rallies, by dint of sheer determination and endurance, he seems determined to try.
What are the other latest developments?
The US Supreme Court ruled on Wednesday that voting officials in Pennsylvania and North Carolina, two key battleground states, could accept postal ballots for several days after the general election in an outcome that cheered Democrats.
In the North Carolina case, the justices allowed lower court decisions to stand that permitted the state board of elections to extend the deadline for accepting ballots to nine days after the vote.
The highest US court also declined to decide before election day whether Pennsylvania election officials can continue receiving votes for three days after the vote. The court's new justice, Amy Coney Barrett, did not take part in either case.
Meanwhile, a study by the US Center for Responsive Politics projected the total cost of this US general election would hit $14bn (£10.7bn), double the cost of the last one.