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 Democrat Joe Biden, which is getting uglier by the hour.

With two months until Election Day, Trump is seizing on fears over violent protests in US cities and questioning the integrity of voting by mail, while challenger Biden intensified his criticism of a "deplorable" leader who he said is not fit for office.

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

In a country on edge over the health crisis and a national reckoning about race, the next 60 days will test the ability of the world's largest economy to organize its election while profoundly changed by a pandemic that has killed 187,000 Americans.

US President Donald Trump took his campaign to key battleground state Pennsylvania as the race heats up US President Donald Trump took his campaign to key battleground state Pennsylvania as the race heats up Photo: AFP / MANDEL NGAN

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 -- which both candidates visited in recent days -- will follow in the coming weeks.

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

Controversy has erupted over US President Donald Trump's comments surrounding his visit to France in November 2018, and his decision not to visit the Aisne-Marne American Cemetery Controversy has erupted over US President Donald Trump's comments surrounding his visit to France in November 2018, and his decision not to visit the Aisne-Marne American Cemetery Photo: AFP / Geoffroy VAN DER HASSELT

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.

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.

US election 2020: Support for Joe Biden and Donald Trump in key battleground states as of September 3 US election 2020: Support for Joe Biden and Donald Trump in key battleground states as of September 3 Photo: AFP / Kun TIAN

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.

The tense campaign battle heated Thursday when The Atlantic magazine reported -- citing four anonymous sources who said they had firsthand knowledge of the discussions -- that Trump had referred to US Marines buried in a World War I cemetery in France as "losers" and "suckers" for getting killed in action.

Democratic presidential candidate Joe Biden has intensified his criticism of Donald Trump, calling him a "deplorable" leader not fit for office Democratic presidential candidate Joe Biden has intensified his criticism of Donald Trump, calling him a "deplorable" leader not fit for office Photo: AFP / JIM WATSON

When visiting France in November 2018 for the centenary of the end of the Great War, Trump did not visit the Aisne-Marne American Cemetery near Paris -- officially because bad weather grounded his helicopter.

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

"The Atlantic magazine is dying, like most magazines, so they make up a fake story in order to gain some relevance," wrote the president, who went on to assail the report as "a disgrace" in a briefing to White House reporters.

Trump's allies launched a swift counteroffensive, flooding social media with images of the president in company of US soldiers -- but others were quick to dig up campaign comments from 2016 when he mocked late senator John McCain, a prisoner of war for years in Vietnam.

The report left Biden as angry as he has appeared on the campaign trail.

"I just think it is sick, it is deplorable, it is so un-American, it is so unpatriotic," Biden said of Trump's quoted remarks, which the Democrat said he believed were accurately reported.

"Who the heck does he think he is?"

Biden turned personal in mentioning his son Beau, a US military veteran and former Delaware attorney general who died of cancer in 2015.

When Beau "went to Iraq for a year, and won the Bronze Star and other commendations, he wasn't a sucker," Biden said.

Trump fired back later from the White house, accusing reporters of playing softball with his Democratic rival.

"I look at the level of question that you people ask, I mean, honestly, it's disgraceful," Trump railed.

"They were like, meant for a child."

Both candidates are expected to remain off the trail during the three-day Labor Day holiday weekend.

But next Friday could see Trump and Biden come face to face, as both have scheduled appearances in Shanksville, Pennsylvania to mark the 19th anniversary of the 9/11 attacks.

Asked if he would share a stage with Trump if invited, Biden said yes: "He is still the president of the United States."