MILWAUKEE – The spotlight will shine firmly on GOP vice presidential nominee Sen. JD Vance on day three of the Republican National Convention.

Vance on Wednesday night will address the roughly 2,400 delegates and thousands of other attendees packed inside Milwaukee's Fiserv Arena, and the millions of Americans watching the GOP convention from home, in his first speech since former President Trump on Monday named the 39-year-old senator from Ohio as his running mate.

"This is clearly the most important speech of JD Vance's career," Dan Eberhart, an oil drilling chief executive officer and a prominent Republican donor and bundler who's attending the convention, told Fox News.

Trump, in making his greatly anticipated and high-stakes running mate announcement as the GOP convention kicked off in swing-state Wisconsin's largest city, will now share the ticket with one of his top supporters in the Senate and a one-time Trump critic who has transformed into a leading America First ally.

TRUMP ANNOUNCES JD VANCE AS HIS RUNNING MATE

Donald Trump applauds as Republican vice presidential nominee J.D. Vance gestures on Day 2 of the Republican National Convention

Republican presidential nominee and former President Donald Trump applauds as Republican vice presidential nominee Sen. J.D. Vance gestures on Day 2 of the Republican National Convention (RNC), at the Fiserv Forum in Milwaukee, Wisconsin, U.S., July 16, 2024.  (REUTERS/Elizabeth Frantz)

The former president and Vance teamed up on Monday and Tuesday nights in the family box above the floor of the GOP convention.

Vance, a former venture capitalist and the author of the bestselling memoir "Hillbilly Elegy" before running for elective office, on Wednesday night will appear on the podium to tell his story.

A source in Vance's political orbit told Fox News to "expect the speech to focus heavily on his bio and incredible life story and how that ties into the America First Agenda."

Another source with knowledge of the speech told Fox News it will "connect his life experiences to the Trump policies. Folding in his firsthand experience of a tough upbringing that shaped his views on a lot of the biggest issues he is passionate about."

The source said those issues include trade, immigration, ending endless wars, fentanyl and drugs, and how inflation hurts the poor the most.

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That story began with Vance growing up in a working-class family in a small city in southwestern Ohio. His parents divorced when he was young, and as his mother struggled for years with drug and alcohol abuse, Vance was raised in part by his maternal grandparents.

After high school graduation, Vance enlisted in the U.S. Marine Corps and served in the Iraq. He later graduated from The Ohio State University and then earned a law degree at Yale University.

Vance, who lives in Cincinnati, moved to San Francisco after law school and worked as a principal in a venture capital firm owned by billionaire venture capitalist Peter Thiel, who later became a major financial supporter of Vance's successful 2022 campaign for the Senate.

J. D. Vance and his wife Usha Vance arrive the first day of the 2024 Republican National Convention at the Fiserv Forum in Milwaukee, Wisconsin

U.S. Sen. JD Vance and his wife Usha Chilukuri Vance look on as he is nominated for the office of Vice President on the first day of the Republican National Convention at the Fiserv Forum on July 15, 2024 in Milwaukee, Wisconsin.   (Anna Moneymaker/Getty Images)

Before running for Senate, Vance grabbed national attention after "Hillbilly Elegy" – which tells his story of growing up in a struggling steel mill city and his roots in Appalachian Kentucky – became a New York Times bestseller and was made into a Netflix film. The story spotlighted the values of many working-class Americans who became supporters of Trump's policies.

Vance was a vocal critic of Trump when the former president first ran for the White House in the 2016 cycle. 

However, Vance eventually supported Trump, praising the former president's tenure in the White House, and in a Fox News interview in 2021, he apologized for his earlier criticism of Trump.

Trump's endorsement of Vance days before the 2022 GOP Senate primary boosted him to victory in a crowded, competitive and combustible race.

"I think the American people are going to love to hear JD’s story of overcoming adversity as a young man, becoming a Marine and serving his country in uniform in Iraq, and going on to becoming a business leader, and now a successful elected leader as well," fellow veteran and fellow Republican Sen. Tom Cotton of Arkansas told Fox News on Tuesday.

Donald Trump and JD Vance react during Day 1 of the Republican National Convention

Republican presidential nominee and former President Donald Trump and Republican vice presidential nominee Sen. J.D. Vance react during Day 1 of the Republican National Convention (RNC), at the Fiserv Forum in Milwaukee, Wisconsin, U.S., July 15, 2024.  (REUTERS/Mike Segar)

Eberhart, asked what Vance needs to accomplish in his address, said that what the senator "needs to do today is show America who he is, why he supports Donald Trump, and why Americans should support Donald Trump."

Democrats, in a taste of things to come, on Monday wasted no time in criticizing Vance.

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President Biden told reporters that Vance was "a clone of Trump on the issues." 

Vice President Harris, in a campaign video released on Wednesday, charged that "Vance will be loyal only to Trump, not to our country."

And the president's campaign argued that Vance was selected because he would "do what [former Vice President] Mike Pence wouldn’t on January 6: bend over backwards to enable Trump and his extreme MAGA agenda, even if it means breaking the law and no matter the harm to the American people."

Get the latest updates from the 2024 campaign trail, exclusive interviews and more at our Fox News Digital election hub.