STORY: Donald Trump on Monday introduced his running mate in the 2024 election: U.S. Senator J.D. Vance.

Trump first revealed the choice on his Truth Social media website, at the start of the four-day Republican National Convention in Milwaukee to nominate the party's presidential ticket.

Vance, who is 39 years old, is a former Trump critic-turned-staunch loyalist, who enjoyed a rapid rise in American politics after winning acclaim for his memoir, Hillbilly Elegy.

Vance himself was harshly critical of Trump before and after Trump's 2016 election win against Democratic presidential nominee Hillary Clinton, calling him an "idiot" and "America's Hitler," among other epithets.

But as Vance geared up to run for the U.S. Senate in Ohio in 2022, he transformed into one of the former president's most consistent defenders. That about-face appeared to win him Trump's endorsement.

VANCE: "Thanks to the president for everything, for endorsing me. I've got to say, a lot of the fake news media out there wanted to write a story that this campaign would be the death of Donald Trump's 'America First' agenda. Ladies and gentlemen, this isn't the death of the America First agenda."

And the Republican would go on to defend Trump's positions.

He has downplayed the severity of the January 6, 2021 attack on the U.S. Capitol by a violent mob of Trump supporters trying to block lawmakers from certifying the results of the 2020 election...

VANCE: "They asked me, they said, well, you know, wasn't January sixth a constitutional crisis?' And I shook my head and I said, no, no, no. January the sixth, Donald Trump trying to challenge an election through constitutional means -that is the opposite of a constitutional crisis."

On the campaign trail, the former venture capitalist has also served as a bridge between Trump associates and wealthy Silicon Valley donors, many of whom have opened their wallets to Trump this election.

Still, Vance's selection has its detractors among Trump associates, notably those who had wanted Trump to select a diverse vice presidential candidate.

Though Trump and Biden are virtually tied in most national polls, Trump trails the Democratic president by significant margins among women and Black Americans.

Some Trump associates privately questioned whether it would be wise to take Vance out of the Senate with Democrats and Republicans vying for control of the upper chamber.

Democrats hold a one-seat advantage, though they are likely to lose ground in the November election.