Donald Trump picked a firebrand US senator as his running mate on Monday night after promising to unite the nation following his attempted assassination.

Trump said JD Vance, 39, one of his fiercest supporters who has been described as an “heir to the Maga movement”, was “best suited” to become vice president if he wins back the White House later this year.

It comes a day after Mr Vance suggested Joe Biden’s rhetoric had “led directly to President Trump’s attempted assassination”.

Trump launched a new “unity” message in the 48 hours after the assassination attempt, pledging that his campaign would “remain resilient in our faith” and not “allow evil to win”.

On Monday night he wrote that Mr Vance would “continue to fight for our Constitution, stand with our troops, and will do everything he can to help me make America great again”.

Meanwhile, Trump received two major boosts on Monday; one of his major criminal trials was dismissed by a judge, and polls showed him gaining rapidly in swing states.

The White House said on Monday that the Secret Service had “failed” to protect Trump during a string of security blunders at Trump’s rally on Saturday, where he was shot in the ear.

Trump’s running mate announcement came on the first day of the Republican national convention in Milwaukee.

The former president also celebrated after a court in Florida dismissed a criminal case against him, in which he was accused of improperly storing classified documents after leaving the White House in 2021.

He said the decision by Aileen Cannon, a judge he appointed, was the “first step” in “uniting the nation after the horrific events on Saturday”, and that other judges considering cases against him should do the same.

Judge Cannon ruled that the appointment of Jack Smith, the government’s special prosecutor, was illegal under the US constitution.

Mr Vance is a strong supporter of Trump’s brand of “MAGA” Republicanism. He rose to prominence after publishing a memoir, Hillbilly Elegy, which explores the life of white working-class Americans. He served as a military journalist in the US Marine Corps during the Iraq War, before being elected as a senator in 2016.

Mr Vance previously criticised Trump as “America’s Hitler” and “cultural heroin”, but later reinvented himself and grew close to the Trump family. He is a prominent sceptic of Western support for Ukraine. He has argued that US support “doesn’t add up,” and that the Biden administration’s position of not negotiating with Vladimir Putin is “absurd”.

On Saturday, after 20-year-old Thomas Matthew Crooks shot Trump, Mr Vance accused the Biden campaign of inciting the violence.

He said the attack was “not just some isolated incident” and had happened because Mr Biden depicted Trump as an “authoritarian fascist who must be stopped at all costs”.

“That rhetoric led directly to President Trump’s attempted assassination,” he said.

Mr Biden’s campaign has since pulled adverts describing Trump as a “dictator” and a “threat” to America.

Responding to Trump’s announcement on Monday, the Biden campaign said Mr Vance would “bend over backwards to enable Trump and his extreme MAGA agenda, even if it means breaking the law and no matter the harm to the American people”.

Karine Jean-Pierre, the White House’s spokesman, said on Monday that the Biden administration would not “politicise” the shooting, and called for America to “unite” after the attack. She claimed there had been no change in the overall direction of Mr Biden’s campaign.

Alejandro Mayorkas, the homeland security secretary, admitted that security officials at the rally had allowed a “direct line of sight” between Trump and a nearby rooftop where Crooks was positioned.

He said an independent review would identify “immediate and longer term corrective actions required to ensure that the no-fail mission of protecting national leaders is most effectively met”.

The review is understood to be investigating why the rooftop, which was around 150 metres from Trump, was outside the secure zone created for his rally.

The Secret Service is also under pressure to explain why warnings from passers-by were not heeded in the minutes before Crooks opened fire.

Security has been increased at the Republican convention, and Mr Mayorkas said the Secret Service would provide protection to Robert F Kennedy Jr, an independent presidential candidate who is also the nephew of John F Kennedy.

Ahead of the convention, Trump said it was a “miracle” that he had survived, by tilting his head to look at a screen in the moment before the bullet struck his ear.

“The doctor at the hospital said he never saw anything like this, he called it a miracle,” he said. “I’m not supposed to be here. I’m supposed to be dead.”

The former president also received a boost in the polls on Monday, as a new survey showed he is just three points behind Mr Biden in Virginia, which Mr Biden won by a considerable margin in 2020.

He is also ahead in Pennsylvania, in a poll conducted by Siena College before the shooting took place.

One senior Democratic congressman effectively admitted defeat, telling the Axios website: “We’ve all resigned ourselves to a second Trump presidency.”