President Donald Trump and challenger Joe Biden traded accusations of graft and clashed on the Covid-19 pandemic Thursday but without landing a knockout blow 12 days before the election in a final debate that many saw as Trump's last big chance to change the narrative.

Perhaps the most startling aspect of the debate in Nashville, Tennessee, turned out to be the relative civility compared to the disastrous first debate last month when Trump spent much of the time shouting frontrunner Biden down.

This time, Trump called his Democratic opponent "Joe" and even lauded the moderator Kristen Welker of NBC News, who had a mute button to keep order, saying: "I respect very much the way you're handling this, so far."

The most heated early exchanges were over mutual accusations of graft.

Trump had signalled he'd try to damage Biden with his pursuit of murky accusations that his son Hunter was involved in graft in China and Ukraine while Biden was vice president under Barack Obama.

Trump, 74, did try to raise the issue repeatedly, saying there were "damning" allegations. He put Biden, 77, on the spot by saying: "I think you owe an explanation to the American people."

Biden flipped the attack, saying no wrongdoing had ever been shown by his family and that serious questions were mounting around Trump himself, including his holding of a bank account in China and failing to publish his US tax returns.

"What they do know is that you are not paying your taxes, or you're paying taxes that are so low," he said, referring to reports on leaked tax data that shows Trump has paid at most $750 in federal income taxes during recent years.

But Biden's heaviest weapon, as throughout his campaign against Trump, was criticism of the president's handling of the coronavirus crisis, which has now killed some 220,000 Americans.

He warned of a "dark winter" coming. "220,000 Americans dead. If you hear nothing else I say tonight, hear this," Biden said, addressing the television audience. "Anyone who's responsible for that many deaths should not remain as president of the United States of America."

- 'Dark winter' -

Trump, who was hospitalized with coronavirus this month but has since recovered, hit back by defending his push to reopen the United States as soon as possible, even when medical experts warn that more caution is needed.

"We're rounding the turn. We're rounding the corner. It's going away," Trump said. "We have a vaccine that's coming, it's ready, it's going to be announced within weeks."

With cases rising rapidly around the country again, a Quinnipiac University poll Thursday found that nearly six in 10 people think the coronavirus is out of control.

Trump and Biden also traded blows on the US leader's friendship with North Korea's Kim Jong Un, which the president said had kept the peace in the Korean peninsula, after Biden and Obama left him "a mess" and the threat of "nuclear war."

Whether the showdown at Belmont University in the country music capital can really shift the election is itself up for debate.

"Both candidates clearly learned important lessons from the inaugural debate that was so poorly received," said Aaron Kall, an expert on presidential debates at the University of Michigan.

"But with only 12 days until the election and tens of millions of Americans early voting, it may be too late to fundamentally alter the upcoming election."

Some 45 million Americans are estimated to have joined an unprecedented wave of early voting and polls indicate that almost all voters have already firmly made up their minds. Biden is steadily ahead, with the Quinnipiac University national poll putting him up at 51 percent to Trump's 41.

Joe Biden denounced President Trump for befriending North Korea's "thug" leader, likening his diplomacy to working with Hitler.

- Kim the thug -

In a sharp clash in their final presidential debate, Biden attacked Trump's insistence that he has avoided war through his summits with Kim Jong Un. "He's talked about his good buddy, who's a thug," Biden said of the young North Korean leader.

"That's like saying we had a good relationship with Hitler before he invaded Europe -- the rest of Europe. Come on."

But Biden indicated he was also willing to meet with Kim, saying his condition would be that Pyongyang works to make the Korean peninsula "a nuclear-free zone."

Trump said that former president Barack Obama had left him "a mess" on North Korea and had warned him of the risk of "nuclear war."

After the summits, "we have a very good relationship. And there's no war," said Trump, who also played down North Korea's recent unveiling of a massive new long-range missile at a military parade.

"He didn't like Obama," Trump said of Kim not meeting the former president. "He didn't like him. He wouldn't do it."

Biden, who was vice president under Obama, hit back that Obama would not meet Kim because he was pushing stronger sanctions.

"President Obama said we're going to talk about denuclearization. We're not going to legitimize you."

Trump first met in June 2018 with Kim in Singapore, the first-ever summit between the countries still technically at war, and later said that the two leaders "fell in love."

The two leaders have met two more times and North Korea has since held off on nuclear and missile tests but analysts say Pyongyang has kept advancing its weapons programs.

Biden holds steady

For a week President Donald Trump has accused Joe Biden of corruption and threatened to unleash devastating accusations about the Democrat's son Hunter during Thursday's debate, but the attacks largely failed to destabilize his White House rival.

The Republican incumbent did raise the murky accusations that Biden profited from corrupt business deals involving his son in Ukraine and China while his father served as vice president under Barack Obama.

But Biden parried the attacks saying "nothing was unethical," and the issue which had the potential to overshadow their debate did not reach a resolution.

Trump did try to poke Biden, saying "I think you owe an explanation to the American people."

The president, trailing in the polls and seeking a jolt that could reverse his fortunes 12 days from Election Day, had relished the opportunity to discuss a laptop that apparently once belonged to Hunter Biden.

Trump said it contained "horrible emails" that showed the extent of the Biden family's formation of a venture with a Chinese company.

"If this is true, he's a corrupt politician, so don't give me the stuff about how you're this innocent baby," Trump said. "Take a look at the laptop from hell."

The computer has been the subject of recent, dubiously-sourced stories in the Rupert Murdoch-owned New York Post. It claimed it had obtained documents from the laptop including emails that showed Hunter Biden introduced his father to an advisor to Burisma, a Ukrainian gas company where Hunter served on the board, in 2015.

Trump also cited unconfirmed emails that addressed the Biden family's business dealings with a Chinese firm, CEFC China Energy Co, that was to invest in the United States and elsewhere.

"Your son said 'We have to give 10 percent to the big man.' Joe, what's that all about?" Trump said. "It's terrible." 

- 'Damning'? -

Biden swatted away the accusations, saying he had "not taken a penny from any foreign source ever in my life."

"My son has not made money in terms of this thing about... China," Biden added, saying it was actually Trump who has profited from business in China, as he pointed to a once-secret Trump bank account there.

Trump, in an apparent attempt to intimidate his onstage rival, invited a former Hunter Biden business associate, Tony Bobulinski, as a debate guest in Nashville.

Hours earlier the man held a press conference where he accused Biden of "involvement" in the financial venture with his son and a Chinese company.

He also said he had evidence on his three personal cell phones that he would be turning over to the FBI on Friday.

"What he said was damning," Trump mentioned at the debate, presumably referring to Bobulinski.

But the attacks lacked the ferocity that Trump showed in their first showdown last month.

And just as Thursday's debate was concluding, The Wall Street Journal -- whose opinion writers this week have fuelled the narrative that Hunter Biden sought to use his father's influence for financial gain -- poured cold water on suggestions that Joe Biden was involved.

"Corporate records reviewed by The Wall Street Journal show no role for Joe Biden," the paper reported.