From Marine to politician: Inside JD Vance's journey to the RNC stage

Freshman Sen. JD Vance stepped onto the national stage at the Republican National Convention Wednesday night to deliver the evening's closing address, letting America get to know the man former President Donald Trump has selected as his vice presidential running mate.

It's a sight that makes his longtime friend Cullen Tiernan proud.

"I always knew that he was smart enough to do whatever he wanted. We've always been attracted to public service, so I really wasn't surprised," Tiernan, a friend of Vance's for more than two decades, told ABC News of his bid to join forces with Trump.

Over the years, Tiernan has had a front-row seat to many of Vance's biggest personal and career milestones, from his wedding to his wife Usha, to his swearing-in as a freshman senator. He says he isn't surprised that his friend is making history as the first post-9/11 veteran on a major party's presidential ticket -- something Tiernan believes will serve Vance well as VP if the Trump/Vance ticket is elected.

"Having a veteran voice like that, somebody who understands what it's like to be an enlisted Marine kind of goes back to his whole 'Hillbilly Elegy' in his story," Tiernan said, referring to Vance's best-selling memoir, which was turned into a film by director Ron Howard. "I think that's going to be really powerful."

Tiernan and Vance's bond began in 2004 when both men were in Defense Information School at Fort Meade, Maryland, both training to become public affairs correspondents in the Marines. The day after they finished training, they were sent to the Marine Corps Air Station in Cherry Point, North Carolina. The two wandered the base together, getting lost but building a budding friendship.

Tiernan remembers watching Vance taking classes for Ohio State while in the barracks, juggling life as a college student and as a Marine. The pair would work together over three years, and their friendship would later be cemented on the battlefields of Iraq .

Tiernan would keep Vance's family updated while Vance went outside the wire in Iraq. When the pair returned to America, Tiernan spent the Fourth of July weekend with Vance's family in Ohio and attended an Ohio State football game.

Vance, if elected in November, would also make history by becoming the first Marine as vice president. Tiernan believes that his friend could convince other left-leaning voters like himself to consider the joint ticket of Trump-Vance in November, especially among military families and working-class voters.

Tiernan, a labor leader in New Hampshire, noted the keynote speech by Teamster's President Sean O'Brien on the first night of the GOP was a good-faith effort to include labor in political discussions. The speech was the first time a Teamsters leader had spoken at the Republican National Convention. Although the union announced it would not be endorsing a presidential candidate in 2024, he believes Vance will continue the outreach to working-class voters in this election.

"In the world of labor politics, I kind of recognize that if you want to accomplish things for working people, it has to be a bipartisan solution for most of the problems and things that we want to improve in people's lives," the New Hampshire labor leader said.

"The good faith effort of including the Teamsters and labor more and more in the conversation and trying to really have a majority of people in Congress who are going to work for working people is my hope, that that's the direction that they're headed in, and then they can bring a lot of people with them."

Tiernan describes his friend as quick-witted, funny, and personable. They would watch "Anchorman" together and the TV series "Arrested Development."

Tiernan says Vance would remain personable despite the difficult challenges of serving in a war zone. The Ohio Senator is the first veteran on a presidential ticket since the late Senator John McCain's White House bid in 2008. He believes the experiences in Iraq "will serve him well, as he is on the campaign trail and whatever else happens next to him."

After Vance's swearing-in ceremony for the Senate, Tiernan, who had spent the day with his friend in Washington, noticed that Vance surrounded himself with old friends from Ohio. Vance told his closest allies that they "were going to help him remember who he is and not to let it get to his head and to stay grounded," Tiernan told ABC News.

"To be a beat away from the White House is another thing, and it's not going to be easy to stay grounded, but I think he's capable of doing it," Tiernan added.