Donald Trump named Senator JD Vance of Ohio as his running mate on Monday, choosing a one-time critic who became a loyal ally and is now the first millennial to join a major-party ticket at a time of deep concern about the advanced age of America’s political leaders.

“After lengthy deliberation and thought, and considering the tremendous talents of many others, I have decided that the person best suited to assume the position of Vice President of the United States is Senator J.D. Vance of the Great State of Ohio,” Mr Trump said in a post on his Truth Social network.

Mr Vance, 39, rose to national fame with the 2016 publication of his memoir, Hillbilly Elegy.

He was elected to the Senate in 2022 and has become one of the staunchest champions of the former president’s “Make America Great Again” agenda, particularly on trade, foreign policy and immigration.

But he is largely untested in national politics and is joining the Trump ticket at an extraordinary moment.

An attempted assassination of Mr Trump at a rally on Saturday has shaken the campaign, bringing new attention to the nation’s coarse political rhetoric and reinforcing the importance of those who are one heartbeat away from the presidency.

Two men in blue suits
Senator JD Vance and Donald Trump (Jeff Dean/AP)

Mr Vance himself faced criticism in the wake of the shooting for a post on X, formerly Twitter, that suggested President Joe Biden was to blame for the violence.

“The central premise of the Biden campaign is that President Donald Trump is an authoritarian fascist who must be stopped at all costs,” Mr Vance wrote.

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

Law enforcement has not yet specified a motivation for the shooting, but the choice is sure to energise Mr Trump’s loyal base.

President Joe Biden said: “He’s a clone of Trump on the issues.”

Speaking to reporters at Andrews Air Force Base shortly before leaving for Nevada for a series of speeches and campaign events, he said: “I don’t see any difference.”

Mr Vance has become a fixture on the conservative media circuit and frequently spars with reporters on Capitol Hill, helping establish him as the kind of leader who could carry Mr Trump’s mantle into the future, beginning with the next presidential election in 2028.

But the choice also means that two white men will now lead the Republican ticket at a time when Mr Trump has sought to make inroads with black and Latino voters.

In Hillbilly Elegy Mr Vance detailed life in Appalachian communities that drifted from a Democratic Party many residents found disconnected from their daily travails.

While the book was a bestseller, it was also criticised for sometimes oversimplifying rural life and ignoring the role of racism in modern politics.

Mr Vance’s fame grew in tandem with Mr Trump’s unlikely rise from a reality television star to Republican presidential nominee and eventually president.

During the early stages of Mr Trump’s political career, Mr Vance cast him as “a total fraud”, “a moral disaster” and “America’s Hitler”.

But like many Republicans who sought relevance in the Trump era, he eventually shifted his tone.

He said he was proved wrong by Mr Trump’s performance in office and evolved into one of his most steadfast defenders.

“I didn’t think he was going to be a good president,” Mr Vance recently told Fox News Channel.

“He was a great president. And it’s one of the reasons why I’m working so hard to make sure he gets a second term.”

Mr Vance was rewarded for his turnaround during his bid for an open Senate seat in 2022, during which he landed Mr Trump’s coveted endorsement and rode it to victory in a crowded Republican primary and a general election hard fought by Democrats.

He is close to Mr Trump’s son Donald Jr.

Mr Vance is now a Trump loyalist who has challenged the legitimacy of criminal prosecutions and civil verdicts against him and questions the results of the 2020 election.

He told ABC News in February that, if he had been vice president on January 6, 2021, he would have told states where Mr Trump disputed Joe Biden wins “that we needed to have multiple slates of electors, and I think the US Congress should have fought over it from there”.

“That is the legitimate way to deal with an election that a lot of folks, including me, think had a lot of problems in 2020,” he said.

Many states adopted emergency measures four years ago to allow people to vote safely during the Covid-19 pandemic.

But judges, election officials in both parties and Mr Trump’s own attorney general have concluded there was no evidence of widespread voter fraud in the 2020 election.

The relationship between the men has been symbiotic.

Mr Vance’s book, subtitled A Memoir of a Family and Culture in Crisis, was embraced for its insights into Mr Trump’s appeal in middle America, where manufacturing job losses and the opioid crisis had driven many families like his into poverty, abuse and addiction.

The tale of Vance’s childhood in Middletown, Ohio, where he was born, and his familial eastern Kentucky hills region also captivated Hollywood.

Ron Howard made it into a 2020 movie starring Amy Adams as Mr Vance’s mother and Glenn Close as his beloved “Mamaw”.

With his grandmother’s encouragement, Mr Vance served in the Marine Corps, including in Iraq, and graduated from Ohio State University and Yale Law School.

From there, he joined a Silicon Valley investment firm before returning to Ohio to launch a non-profit organisation that he said would aim to develop opioid addiction treatments that might be “scaled nationally”.

Ultimately, Our Ohio Renewal failed closed down.

During the 2022 campaign, Tim Ryan, his Democratic rival, said the charity was little more than a front for Mr Vance’s political ambitions.

Mr Ryan pointed to reports that the organisation made payments to a Vance political adviser and conducted public opinion polling, even as its actual efforts to address addiction largely floundered.

Mr Vance denied the charges.

As a senator, Mr Vance has shown some willingness to work across the aisle.

He and Ohio’s senior senator, Democrat Sherrod Brown, have teamed up on a number of issues important to the state, including fighting for funding for a chip facility Intel is building in central Ohio and introducing rail safety legislation in response to a fiery derailment in East Palestine, Ohio.