Milwaukee, Wisconsin - JD Vance bashed President Joe Biden, assailed the media, and saluted Donald Trump's bravery Wednesday in remarks ahead of a keynote address to grassroots Republicans to accept his nomination as the party's vice-presidential candidate.

Speaking on the sidelines of the Republican National Convention – his first campaign stop as Trump's running mate – the US senator applauded Trump's reaction to cheating death at the hands of a gunman in Pennsylvania at the weekend.

"The contrast between the lie that the media tells about President Trump and the man that all of us actually know, of course, we saw it in stark definition on Saturday," Vance told the audience in Milwaukee.

Vance said he was "terrified that we had just lost a great president" when a volley of gunfire forced Trump to hit the stage floor at the rally in Butler.

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"And then, of course, he stands up a minute later, after they shot him – they literally shot him – and he raises his fist in the air and he says, 'Fight! Fight! Fight!"

Vance – seen by critics as a conspiratorial, divisive firebrand – didn't explain whom he meant by "they," but he had immediately blamed Democrats in the aftermath of the attack.

Authorities have not suggested any motive for the shooting as they investigate the slain assailant, and there is no suggestion that anyone else was involved.

Vance (39) took the opportunity to plug the Trump brand, which he described as "synonymous with luxury and with beauty in the real estate world."

And he contrasted the economy under "real president Donald Trump" with inflation under "Fake Scranton Joe Biden," saying that under the Democrat, "the basic trappings of middle-class life have become less and less attainable."

"Let's get rid of him, and let's bring Donald Trump back to the White House," he said.

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Vance was to take center stage at the convention later Wednesday as Trump bids for a stunning return to the White House, four years after losing to Biden.

The one-term senator would be the third-youngest vice president in history – and one of the least experienced – if 78-year-old Trump prevails in his rematch against Biden in November.

The way the best-selling writer talks about Trump is in marked contrast to the hostility he voiced as he toured television studios in 2016 with a book to sell.

Vance was an uncompromising "Never Trumper" at the time of his new boss's election, labeling the hard-right tycoon "a moral disaster" and comparing him to Adolf Hitler.

He reinvented himself when he entered politics and won Trump's endorsement in the 2022 Ohio Senate race.

Vance has since grown into a cheerleader-in-chief for Trump's anti-immigrant rhetoric and isolationist foreign policy – including opposition to US support for Ukraine's war against Russian invasion.

Despite making his name with the 2016 memoir Hillbilly Elegy, a best-selling account of his Appalachian family and modest Rust Belt upbringing, Vance remains something of an unknown quantity to the wider public.

He is slated to speak for around half an hour from 9:30 PM, in a speech expected to lean heavily on his biography and relating elements of his life story to the experience of ordinary Americans.

Vance is already a hit with the party faithful in Milwaukee and was rewarded with a rowdy ovation as he stepped on the convention floor Monday with his wife, Usha, who is due to speak just before him.

Some 50,000 Republicans have descended on the shores of Lake Michigan for the four-day convention, where the Pennsylvania shooting – which killed one bystander and left the ex-president with a bloodied ear – has dominated proceedings.

The convention has emphasized party unity behind Trump, banishing the torrent of scandals that range from his role in attempts to overturn the 2020 election to his criminal conviction in a New York court this May.