(NewsNation) — Sen. JD Vance, R-Ohio, who was once one of former President Donald Trump’s fiercest critics, is entering the 2024 election as his running mate.

Donald Trump announced his pick on Truth Social Monday as the Republican National Convention kicked off in Milwaukee.

“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,” Trump posted on Truth Social.

Trump had previously said his top consideration for a vice president is whether someone is qualified to take over as commander in chief.

Who is J.D. Vance?

James David Vance is a Republican U.S. Senator from Ohio.

Swept to national prominence by his bestselling memoir, “Hillbilly Elegy,” Vance has held office for less than two years. But during his short time in the Senate, the former venture capitalist from Ohio has established himself as one of the fiercest defenders of Trump’s “Make America Great Again” agenda, especially when it comes to foreign policy, trade and immigration.

Despite his early criticism of Trump, Vance has become personally close with the former president and his son Donald Trump Jr., who has talked up the senator.

Vance has become a fixture of the conservative media circuit, frequently spars with reporters on Capitol Hill and has appeared with Trump at recent fundraisers and at court.

After high school, Vance joined the U.S. Marine Corps and served in Iraq. He then attended Ohio State University and later Yale Law School, where he graduated in 2013.

Vance is married to Usha Chilukuri Vance, a law school classmate whom he met at Yale. They have three children together. Before entering politics, Vance worked in the technology industry in Silicon Valley and later moved back to Ohio to start his own investment firm.

Vance’s path to Congress

Vance’s journey to Congress began with his rising profile as a conservative voice. His memoir catapulted him into the national spotlight. The book’s exploration of working-class issues and cultural decline in the Rust Belt resonated with many readers and policymakers.

In 2021, Vance announced his candidacy for the U.S. Senate seat being vacated by retiring Republican Sen. Rob Portman. Despite initial skepticism from some Republicans due to his past criticisms of Trump, Vance secured Trump’s endorsement during the primary. This endorsement proved crucial in a crowded Republican field.

In the general election, Vance faced Democratic candidate Tim Ryan. The race was closely watched nationally, with Vance ultimately winning and taking office in January 2023.

Vance and Trump

Vance’s relationship with Trump has been complex.

In 2016, Vance was one of Trump’s fiercest critics, casting the then-reality TV star as “a total fraud” and “moral disaster” and calling him “America’s Hitler.”

However, as Trump’s influence in the Republican Party grew, Vance’s stance shifted.

By the time of his Senate run, Vance had become a strong supporter of Trump and his policies. Trump’s endorsement of Vance in the 2022 Ohio Republican primary was seen as a major factor in Vance’s victory.

Since entering the Senate, Vance has generally aligned himself with Trump’s political agenda and style. This evolving relationship and Vance’s appeal to working-class voters in a key swing state could make him an attractive vice presidential candidate for Trump in 2024.

At 39, Vance will inject some millennial energy into a race that features an 81-year-old (Biden) and a 78-year-old (Trump) at the top of the major parties’ tickets.

Vance has said he was proved wrong by Trump’s performance in office, and the senator now castigates the liberals who made his book a bestseller as they sought a window into understanding Trumpism.

The senator also has strong connections with donors in Silicon Valley and elsewhere.

Vance is not even 18 months into his first term in public office, and voters may question his experience.

Vance’s political stances

Vance is opposed to abortion. He declared himself “100% pro-life,” according to his 2022 campaign website for the Senate. As far as exceptions for rape and incest, Vance told The Washington Post in 2021, “Two wrongs don’t make a right.”

His policy has appeared to change in recent months, with the lawmaker telling CNN in December 2023 that the Republican Party must “accept that people do not want blanket abortion bans. They just don’t. And I say that as a person who wants to protect as many unborn babies as possible. We have to provide exceptions for life of the mother, for rape, and so forth.”

In his first interview since being named to the ticket, Vance told Fox News’ Sean Hannity he stands with Trump’s position that abortion rights should be left to the states.

“It is reasonable to let voters in states make those decisions,” Vance said. “Doesn’t mean we have to agree with it, but you have to have some respect for the political process.”

Trump’s campaign partner has built a brand on economic populism, diverting from the GOP’s traditional pro-business platform.

As a Marine Corps veteran, Yale Law School graduate and former venture capitalist who grew up in poverty in Rust Belt Ohio, Vance partnered with Sen. Elizabeth Warren, D-Mass., to crack down on big banks.

Abroad, Vance has made his stance on the war in Ukraine clear, opting to reject any U.S. aid for the country. He was also a key figure in the unsuccessful attempt to block a $60 billion supplemental aid package to Ukraine.

“I think it’s ridiculous that we’re focused on this border on Ukraine,” Vance said in a podcast interview with Steve Bannon. “I’ve got to be honest with you, I don’t really care what happens to Ukraine one way or the other.”

Impact on his Ohio seat

If Vance were to win the vice presidency, it would create a vacancy in his Senate seat. According to Ohio law, the governor would have the power to appoint a temporary replacement to serve until the next general election.

Gov. Mike DeWine, a Republican, would be responsible for selecting Vance’s replacement. This could potentially allow DeWine to choose a more moderate Republican, which might shift the ideological balance of the seat.

Ohio voters would have the opportunity to elect a new senator to complete Vance’s term in the next general election. This could lead to a competitive race, potentially altering the balance of power in the closely divided U.S. Senate.

The Associated Press, NewsNation partner The Hill and NewsNation’s Tanya Noury contributed to this story.