The US election is officially open: North Carolina on Friday launched vote-by-mail operations for the November 3 contest between President Donald Trump and Joe Biden, which is getting uglier by the day.

Worries about the unabated spread of the coronavirus are expected to prompt a major increase in the number of ballots cast by mail, as Americans avoid polling stations.

In a country on edge over the health crisis and a national reckoning about race, the next two months will test the ability of the world's largest economy to organize its election profoundly changed by the pandemic.

North Carolina was to begin mailing out more than 600,000 ballots, in response to a major spike in demand.

Other key battleground states such as Wisconsin will follow in the coming weeks.

How Americans vote has become another flashpoint in an increasingly divisive political landscape.

A recent poll by USA Today and Suffolk University showed that 56 percent of Republican voters surveyed said they would go in person to vote, while only 26 percent of Democrats plan to do the same.

One out of every four Biden voters said that if the Democratic former vice president loses on November 3, they would not be ready to accept a Trump victory as "fairly won." About one in five Trump voters said a similar thing.

Seeds of doubt 

As he seeks a second four-year term, Trump has for months sowed doubt among his base about the legitimacy of an election with a significant number of mail-in votes.

Trump -- who himself votes by mail in his adopted home state of Florida -- has repeatedly, and without evidence, said mail-in voting could lead to widespread fraud.

He even has suggested to his supporters that they try to vote twice to test the system -- earning him scorn on social media and from Democrats.

Facebook, the world's largest social network, reminded users that voting by mail has a "long history of trustworthiness" in the United States, as does voting in person.

'Losers' 

The tense campaign battle between Trump and Biden heated up Thursday when The Atlantic magazine reported that the Republican incumbent had referred to US Marines buried in a World War I cemetery in France as "losers" and "suckers" for getting killed in action.

The backlash was swift, and Trump sent a barrage of tweets Thursday and another on Friday to defend himself.

"The Atlantic magazine is dying, like most magazines, so they make up a fake story in order to gain some relevance," Trump wrote.

"Story already refuted, but this is what we are up against."

The story refers to a visit to France made by Trump in November 2018, for the centenary commemorations of the end of the Great War.

Trump did not visit the Aisne-Marne American Cemetery near Paris, officially because his helicopter could not take him there due to bad weather.

But The Atlantic, citing four anonymous sources who said they had firsthand knowledge of the discussions, reported that he had told senior staff: "'Why should I go to that cemetery? It's filled with losers'."

On the way back to Washington from a campaign rally in Pennsylvania, Trump erupted, slamming the allegations as "disgusting, grotesque, reprehensible lies."

"I would be willing to swear on anything that I never said that about our fallen heroes," he said.

His top aides quickly launched a counteroffensive, flooding social media with pictures of the president with US soldiers.

But excerpts from his campaign speeches in 2016, when he mocked late senator John McCain -- a prisoner of war for years in Vietnam -- also quickly returned to the fore.

Swing states 

On Friday, both Trump and Biden were staying close to home.

The former vice president was to speak Friday in his home state of Delaware on "the economic crisis that has been worsened by Trump's failure to get the virus under control," his campaign said.

Trump was not expected to stray far from the White House during the three-day Labor Day holiday weekend.

With two months to go until Election Day, all eyes are turning to the key swing states that could tip the election one way or the other.

Pennsylvania and Florida -- both won by Trump in 2016 in tight contests over Hillary Clinton -- are particularly in the spotlight.

According to the latest poll by Quinnipiac University, Biden has a solid lead in Pennsylvania, with 52 percent support to 44 percent for Trump.

In Florida, the two men are running neck-and-neck, with 48 percent for the Democratic challenger and 45 percent for the incumbent.