Two days after surviving an assassination attempt, Donald Trump has named his running mate as JD Vance – a firebrand Ohio senator who penned a bestselling memoir about growing up in poverty.

Trump revealed his pick for US vice-president on social media on Tuesday (AEST) at the start of the Republican National Convention in Milwaukee.

It came as Trump was handed another major legal victory, when a federal judge dismissed the criminal case against him relating to classified documents.

Aged 39, James Daniel Vance, a father of three and Yale graduate, is the author of bestselling memoir Hillbilly Elegy, which was turned into a movie directed by Ron Howard.

He was once a fierce Trump critic, calling him an “idiot” and “America’s Hitler”, but is now a staunch defender.

Vance could increase the odds of supporters turning out for the November 5 US presidential election as the Ohio native is deeply popular with the Republican candidate’s base.

Vance was officially nominated as vice-presidential candidate at the convention on Tuesday (AEST) after Trump’s announcement.

JD Vance, with wife Usha, at he Republican National Convention in Milwaukee. Photo: Getty

Trump posted that he had considered the “tremendous talents of many others” before choosing Vance.

“JD honourably served our Country in the Marine Corps, graduated from Ohio State University in two years, Summa Cum Laude, and is a Yale Law School Graduate, where he was Editor of The Yale Law Journal, and President of the Yale Law Veterans Association,” he wrote on Truth Social.

He said JD’s book “championed the hard-working men and women of our country”.

“JD has had a very successful business career in Technology and Finance, and now, during the Campaign, will be strongly focused on the people he fought so brilliantly for, the American Workers and Farmers in Pennsylvania, Michigan, Wisconsin, Ohio, Minnesota, and far beyond….”

The four-day Republican convention opened in downtown Milwaukee on Monday, two days after Trump narrowly survived an assassination attempt in Pennsylvania.

Trump is due to formally accept the party’s nomination in a prime-time address on Thursday and will challenge Democratic President Joe Biden in the November 5 election.

He has been the presumptive nominee for months but it was the vote of RNC delegates in Milwaukee that made it official on Tuesday.

Inside the convention hall, delegates flanked by “Make America Great Again” signs cheered wildly as state after state voted their support for Trump’s second term.

Major legal victory

Earlier, a Florida judge dismissed the criminal case against Trump relating to illegally keeping classified documents after leaving office.

Trump-appointed District Judge Aileen Cannon ruled that the prosecution’s chief counsel, Jack Smith, was unlawfully appointed to the role and did not have authority to bring the case.

Trump, in a social media post, said Cannon’s ruling should be a “just the first step” and called for the dismissal of all four criminal cases against him.

Prosecutors are likely to appeal Cannon’s ruling.

The judge found that US Attorney-General Merrick Garland, who named Smith in 2022 to oversee investigations involving Trump, had no authority “to appoint a federal officer with the kind of prosecutorial power wielded by Special Counsel Smith”.

Cannon also found that Smith’s investigation had been improperly funded through a permanent and unlimited fund Congress set aside in the 1980s for independent investigations.

It follows the US Supreme Court ruling on July 1 that Trump cannot be prosecuted for actions that were within his constitutional powers as president.

Cannon’s ruling came two days after Trump was the target of an assassination attempt at a campaign rally in western Pennsylvania.

‘Unite our country’: Trump

Trump said the reality was sinking in after he survived an attack by a 20-year-old shooter at his campaign rally in Pennsylvania on Sunday (AEST).

One shot apparently hit Trump’s upper right ear, leaving his face streaked with blood, but he was not severely wounded. His campaign said he was doing fine.

“That reality is just setting in,” Trump told the Washington Examiner.

“I rarely look away from the crowd. Had I not done that in that moment, well, we would not be talking today, would we?”

Trump said it was either “by luck or by God” that he survived.

“I’m not supposed to be here. I’m supposed to be dead,” he told the New York Post during the same interview.

One person in the crowd was killed and two others wounded before Secret Service agents fatally shot the gunman.

Trump and Biden have urged calm and unity, aiming to lower temperatures in a country whose a deep political divide has grown even more pronounced during the presidential race.

Trump pumped his fist in the air several times as he descended the stairs from his plane after arriving in Milwaukee.

“This is a chance to bring the whole country, even the whole world, together. The speech will be a lot different, a lot different than it would’ve been two days ago,” he told the Washington Examiner.

“I want to try to unite our country,” he told the New York Post during the flight to Milwaukee. “But I don’t know if that’s possible. People are very divided.”

The FBI named Thomas Matthew Crooks of Bethel Park, Pennsylvania, as the suspect in what it called an attempted assassination. He was a registered Republican, according to state voter records and had made a $US15 donation to a Democratic political action committee at the age of 17.

FBI officials said on Sunday the shooter acted alone. The agency said it had yet to identify an ideology linked to Crooks or any indications of mental health issues. Nor had it found any threatening language on Crooks’ social media accounts.

Biden, a Democrat, ordered a review of how the gunman, who was shot dead by agents moments after opening fire, could have taken up an elevated position so close to Trump, who as a former president has lifetime protection by the US Secret Service.

Biden said the assassination attempt was “contrary to everything we stand for as a nation”.

“There is no place in America for this kind of violence,” he said at the White House.

Republican National Convention chairman Michael Whatley said on Fox News that authorities had worked to safeguard the venue for the Republican convention. Officials have spent months in security preparations.

The Secret Service denied accusations by some Trump supporters that it had rejected campaign requests for additional security.

“The assertion that a member of the former president’s security team requested additional security resources that the US Secret Service or the Department of Homeland Security rebuffed is absolutely false,” spokesperson Anthony Guglielmi said.

“In fact, recently the US Secret Service added protective resources and capabilities to the former president’s security detail.”

The attack was the first shooting of a US president or major party candidate since the 1981 attempted assassination of Republican president Ronald Reagan.

-with AAP/DPA