Former President Donald Trump selected Senator J.D. Vance from Ohio as his running mate on Monday, choosing a former critic who has become a loyal ally and is now the first millennial to join a major party ticket at a time of deep concern about the aging of political leaders in America.

Trump said in his speech, "After lengthy discussions and careful considerations, and taking into account the tremendous talents of many others, I have decided that the best person to serve as Vice President of the United States is Senator J.D. Vance from the great state of Ohio." This was posted on his social media network as the Republican National Convention convened in Milwaukee.

Vance, who is 39 years old, rose to national prominence with his 2016 memoir "Hillbilly Elegy." He was elected to the Senate in 2022 and has become one of the strongest advocates for the former president's "Make America Great Again" agenda, especially regarding trade, foreign policy, and immigration.

However, he has not been significantly tested in national politics and joins Trump’s ticket at an extraordinary moment. The assassination attempt on Trump during a rally on Saturday shook the campaign, bringing new attention to the rough political discourse in the country and underscoring the importance of those who are a heartbeat away from the presidency.

Vance has faced criticism following the shooting for a post on X suggesting that President Joe Biden is responsible for the violence.

Vance wrote, “The fundamental premise of Biden's campaign is that President Donald Trump is a fascist authoritarian who must be stopped at all costs.” He added that “this rhetoric directly led to the assassination attempt on President Trump.”

Law enforcement authorities have not yet identified a motive for the shooting.

The choice energizes Trump's loyal base

Trump stated that Vance “will focus intensely on the people he has fought bravely for, the working men and women of Pennsylvania, Michigan, Wisconsin, Ohio, Minnesota, and beyond.” Many of these Midwestern states are expected to play a crucial role in the upcoming November elections.

Trump and Vance spoke about 20 minutes before the Truth Social post when Trump officially offered him the job, according to a person familiar with the call who asked for anonymity to discuss the private conversation.

Florida Senator Marco Rubio and North Dakota Governor Doug Burgum, the other two competitors on Trump’s shortlist, were informed earlier on Monday afternoon that they were not his choice, according to people familiar with their discussions.

Discussions over the past ten days between Rubio and Trump’s campaign focused on concerns about residency issues and how to handle the two men residing in the same state, according to a person familiar with the private talks who insisted on anonymity to discuss them.

The source said Trump’s campaign wanted to be 100 percent sure that there would not be a prolonged legal battle over this issue, and Rubio was not willing to uproot his family.

Trump spent months testing the waters, evaluating his competitors’ performances on television, in fundraising, and at rally stages. Many, including Burgum and Vance, joined him in his criminal trial related to hush money in New York. Others were present at last month's debate, where Biden's disastrous performance turned his campaign upside down, leading to widespread calls for him to step aside for a younger candidate.

This choice is sure to invigorate Trump's loyal base. Vance has become a staple in the conservative media sphere and often engages in arguments with reporters on Capitol Hill, helping to establish himself as a kind of leader who could carry Trump’s mantle in the future, starting with the next presidential election in 2028.

But the choice also means that two white men will now lead the Republican party ticket at a time when Trump seeks to make inroads with Black and Latino voters.

In "Hillbilly Elegy," Vance explains the specifics of life in Appalachian communities that have drifted away from the Democratic Party, where many residents found themselves disconnected from their everyday struggles. Although the book was a bestseller, it has also faced criticism for oversimplifying rural life at times and overlooking the role of racism in modern politics.

Vance was once one of Trump’s fiercest critics

Vance’s fame rose alongside Trump’s unexpected ascent from reality TV star to Republican presidential candidate and eventually president. During the early stages of Trump’s political career, Vance described him as “completely fraudulent,” a “moral catastrophe,” and “America’s Hitler.”

But like many Republicans seeking to stay connected to the Trump era, Vance eventually changed his tone. He said Trump’s performance in office proved him wrong and evolved to become one of his most steadfast defenders.

Vance recently told Fox News, “I didn’t think he would be a good president. He turned out to be a great president. That’s one of the reasons I’m working hard to make sure he gets a second term.”

Vance was rewarded for his shift during his attempt to secure an open Senate seat in 2022, where he received the coveted endorsement from Trump and led him to victory in a crowded Republican primary and the general election that Democrats struggled in. He is close to Trump’s son Donald Jr.

Vance is now one of Trump’s loyalists who has challenged the legitimacy of criminal and civil prosecutions against him and questioned the results of the 2020 election.

He told ABC News in February that if he had been the vice president on January 6, 2021, he would have told the states where Trump opposed Biden's victory “that we need multiple voter lists, and I think Congress should have had multiple election rolls. Fight it out from there.”

He said, “This is the legitimate way to deal with elections that many people, myself included, believe faced a lot of problems in 2020.”

Many states enacted emergency measures four years ago to allow people to vote safely during the COVID-19 pandemic. But judges and election officials from both parties, as well as Trump’s attorney general, concluded that there was no evidence of widespread voter fraud in the 2020 election.

Vance’s book — titled "A Memoir of Family and Culture in Crisis" — was embraced for its insight into Trump’s appeal in Middle America, where job losses in manufacturing and the opioid crisis have pushed many families like his into poverty, abuse, and addiction.

Vance's difficult childhood story in Middletown, Ohio, where he was born, and the family’s Eastern Kentucky hills also captivated Hollywood. Ron Howard turned it into a 2020 film starring Amy Adams as Vance's mother and Glenn Close as the beloved "Mamaw."

Encouraged by his grandmother, Vance went on to serve in the Marine Corps, including in Iraq, and graduated from Ohio State University and Yale Law School. From there, he joined a venture capital firm in Silicon Valley before returning to Ohio to launch a nonprofit he said was aimed at developing opioid addiction treatments that could be "scaled nationally."

Ultimately, our Ohio Renewal program failed in that mission and was closed down. During the 2022 campaign, it was the U.S. Rep. Tim Ryan, Vance's Democratic opponent, who accused the charity of being nothing more than a front for Vance’s political ambitions. Ryan pointed to reports that the organization paid money to a political consultant for Vance and conducted public opinion polling, even as its actual efforts to address addiction largely stalled. Vance denied the characterization.

As a senator, Vance has shown some willingness to work across the aisle. He and Ohio’s senior senator, Democrat Sherrod Brown, have collaborated on several important issues for the state, including the fight for a $20 billion chip facility being built by Intel in central Ohio and introducing rail safety legislation in response to derailments.