By MICHELLE L. PRICE, WILL WEISSERT and JILL COLVIN Associated Press

MILWAUKEE (AP) — Donald Trump ’s running mate JD Vance will introduce himself to a national audience Wednesday as he addresses the Republican National Convention.

The Ohio senator’s headlining address will be his first speech as the Republican vice-presidential nominee. He’s a relative political unknown who rapidly morphed in recent years from a severe critic of Trump to an aggressive defender.

Vance, 39, is positioned to become the next potential leader of the former president’s political movement, which has reshaped the Republican Party and busted many longtime political norms. The first millennial to join a major party ticket, he joins the race when questions about the age of the men at the top of the tickets — 78-year-old Trump and 81-year-old President Joe Biden — have been high on the list of voters’ concerns.

Trump, as the presidential nominee, is expected to speak Thursday, the convention’s final night.

Vance is expected to lean into his biography, as someone who grew up in hardscrabble Kentucky and Ohio and became a Marine, an Ivy League graduate, a businessman and a bestselling author with his memoir, “Hillbilly Elegy.” The book exploring his blue-collar roots made him a national name when it was published in 2016 and was seen as a window into some of the cultural forces that propelled Trump to the White House.

Vance, though, was a harsh critic of Trump at the time, referring to him in interviews as “noxious” and someone who “is leading the white working class to a very dark place.” He even once referred to him as “America’s Hitler.”

He began warming to Trump over the years, especially as he sought in 2022 to run for the U.S. Senate. Vance won Trump’s endorsement, which helped him secure the party’s nomination for the Ohio Senate seat.

Vance has become one of Trump’s most aggressive defenders as the former president has sought the office a third time, sparring with journalists, campaigning on his behalf and appearing with the candidate at his trial in New York.

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In his first interview after accepting Trump’s offer to join the ticket, Vance sought to explain his metamorphosis. Vance said in a Fox News Channel interview Monday that Trump was a great president and changed his mind.

“I think he changed the minds of a lot of Americans, because again he delivered that peace and prosperity,” Vance said.

Donald Trump Jr., the former president’s son and a close friend of Vance, is also slated to speak Wednesday, according to a person close to Trump Jr. who spoke on condition of anonymity because the official speaking schedule has yet to be released.

Beyond Vance’s prime-time speech, the Republican Party intends to focus on a theme of American global strength.

Republicans contend that the country has become a “global laughingstock” under Biden’s watch and are expected to make a case Wednesday hitting on their theme to “Make America Strong Once Again.” That’s expected to include Trump’s “America First” foreign policy that redefined relationships with some allies and adversaries.

Democrats have sharply criticized Trump — and Vance — for questioning U.S. support for Ukraine in its defense against Russia’s invasion.