Las Vegas, Nevada - President Joe Biden hits the campaign trail Tuesday for the first time since the assassination attempt against Donald Trump, seeking a balance between criticizing his rival and heeding his own calls to cool down American politics.

As Trump rides a wave of support at the Republican National Convention in Milwaukee, Democrat Biden – who is trying to salvage his re-election bid – is in Las Vegas to reach out to a key constituency of Black voters at the NAACP advocacy group.

The 81-year-old president will also record an interview with Black broadcaster BET, the latest in a series of one-on-ones as he works to ease concerns over his age and mental acuity following his dismal debate performance against Trump last month.

Amid calls for Americans to "lower the temperature" of politics in the wake of the attempt on Trump's life, Biden canceled a campaign trip to Texas on Monday but continued with his Nevada visit.

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Biden narrowly won Nevada in his 2020 election victory over Trump.

The trip comes as Democrats push ahead with plans to rush through Biden's own nomination ahead of the party's conference in August, despite calls to postpone it while the party remains torn over his candidacy.

While the attempt on Trump's life has briefly silenced the drumbeat of Democrats who called on Biden to step aside, tensions are still bubbling beneath the surface as the president trails in the polls.

Biden's campaign insisted the virtual roll-call was necessary before the August 19 convention as Republican-led Ohio had moved its filing date to August 7, otherwise Biden risked not appearing on the ballot.

"There have been virtual roll calls in previous presidential elections" Democratic deputy campaign manager Quentin Fulks told a press conference on the sidelines of the Republican gathering in Milwaukee.

"Ohio Republicans decided to play games... it is our obligation as a campaign to make sure that President Biden is on the ballot."

The move is, however, only likely to fuel questions about whether Biden can win a second term after a faltering showing at the debate where he mangled his words and lost his train of thought.

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Biden, who says he "screwed up" due to jet lag and illness, forcefully defended his fitness for the job in a second major US network interview on Monday night.

"I'm old," Biden told NBC. "But I'm only three years older than Trump, number one. And number two, my mental acuity has been pretty damn good."

Biden also defended his rhetoric about Trump after Republicans accused him of stocking divisions ahead of the shooting in which the former president's ear was hit.

FBI investigators are still probing 20-year-old gunman Thomas Matthew Crooks' motivation for the attack on a Trump rally in Butler, Pennsylvania. A bystander was killed, and Crooks was shot dead.

Biden admitted it was a "mistake" to tell donors in a call last week that it was "time to put Trump in the bullseye" of his election campaign.

But Biden doubled down on what he said was the need to "talk about the threat to democracy" posed by former president Trump.