• Donald Trump has selected Sen. JD Vance of Ohio as his presidential running mate.
• Vance was later formally selected as Trump's running mate, during the first day of the Republican National Convention in Milwaukee, Wisconsin.
• Doug Burgum and Marco Rubio have both been told that Trump will not pick them at the RNC, sources told NBC.

Donald Trump has selected Sen. JD Vance of Ohio as his presidential running mate, ending months of speculation about the Republican nominee's choice to help him challenge President Joe Biden and Vice President Kamala Harris.

"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 said Monday in a Truth Social post.

Vance was formally selected as Trump's running mate later Monday afternoon, during the first day of the Republican National Convention in Milwaukee, Wisconsin.

Trump was officially picked as the GOP's presidential nominee earlier in the day.

Trump's selection provides a sudden, massive jolt in stature for the 39-year-old Vance, who joined the Senate as a political newcomer less than two years ago.

It's also the culmination of a long-term shift toward Trumpism for Vance, who was once a vocal critic of Trump.

Vance gained fame in 2016 through his bestselling memoir "Hillbilly Elegy," which traced his rural upbringing in Ohio and mused on the culture and politics of Appalachia.

While not without its critics, the book quickly earned Vance a reputation as a trenchant political analyst who, despite an Ivy League education, possessed a unique sense of how the White working class viewed the rest of the country.

In the private sector, Vance worked for Mithril Capital, the venture-capital firm run by Peter Thiel, and started his own VC firm, Narya, in 2019.

Vance ran in 2022 for the U.S. Senate seat being vacated by retiring Sen. Rob Portman, a Republican. Vance beat former Rep. Tim Ryan, D-Ohio, by a 53%-47% margin, and took office in January 2023.

In an interview with Fox News' Sean Hannity that aired Monday night, Vance described receiving the call from Trump earlier in the day asking him to be his running mate.

"He just said 'Look, I think that we got to go save this country. I think you're the guy who can help me in the best way. You can help me govern. You can help me win. You can help me in some of these midwestern states like Pennsylvania, Michigan, and so forth,'" Vance said Trump told him.

Two other top Republican vice presidential contenders, Sen. Marco Rubio of Florida and North Dakota Gov. Doug Burgum, were told earlier that they would not be picked for the role, NBC News reported.

The Biden campaign promptly panned the selection, accusing Trump of picking Vance because he will "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."

"Billionaires and corporations are literally rooting for J.D. Vance: they know he and Trump will cut their taxes and send prices skyrocketing for everyone else," read the statement from Biden-Harris campaign chair Jen O'Malley Dillon.

Harris has previously accepted an invitation from CBS News to participate in a vice presidential debate on either July 23 or Aug. 13.

After Trump revealed his selection, Harris left Vance a message congratulating him and welcoming him to the race, a source familiar with the call told NBC.

Shortly after Vance was announced as Trump's running mate, the senator's wife, Usha Vance, resigned from Munger, Tolles & Olson, the law firm where she worked as an attorney.

"Usha has informed us she has decided to leave the firm," according to a statement from the firm shared with CNBC. "Usha has been an excellent lawyer and colleague, and we thank her for her years of work and wish her the best in her future career."

Before entering politics, Vance had slammed Trump as a "total fraud" and even compared him and his MAGA political movement to a harmful drug.

"Trump's promises are the needle in America's collective vein," Vance wrote in The Atlantic before Trump won the 2016 election.

But as a politician, Vance has morphed into one of the most loyal and extreme backers of both Trump and his brand of nationalist, populist politics.

In his interview with Hannity, Vance said he was not trying to "hide from" his prior criticism of Trump.

"I was certainly skeptical of Donald Trump in 2016," Vance told Hannity. "But President Trump was a great president, and he changed my mind. I think he changed the minds of a lot of Americans, because again, he delivered that peace and prosperity."

In recent months, Vance has been quick to display his new pro-Trump stance.

He was among the parade of Republicans who appeared outside of Trump's criminal hush money trial in New York City to decry the prosecution of the GOP leader.

He later claimed the trial was "election interference," and that its "main goal" was "psychological torture" against Trump. The jury in that trial convicted Trump on 34 counts of falsifying business records; Trump is currently set to be sentenced on Sept. 18.

After Trump survived a shocking assassination attempt at a campaign rally in Pennsylvania over the weekend, Vance baselessly blamed the Biden campaign for the attack.

"The central premise of the Biden campaign is that President Donald Trump is an authoritarian fascist who must be stopped at all costs," Vance wrote within hours of the shooting. "That rhetoric led directly to President Trump's attempted assassination."

The attack, which left one rally attendee dead and Trump with a minor injury, sent shockwaves across the country and spurred condemnations of violence across the political aisle.

Biden, in an Oval Office address after the Trump rally shooting, urged Americans to lower the temperature of political rhetoric and reaffirm the democratic norms of civil disagreement and decency.

As a senator, Vance has opposed sending U.S. aid to Ukraine as it fights against invading Russian forces, and he has repeatedly voted against legislation that would preserve or expand federal abortion rights.

The Vance announcement added to an already eventful day of Trump-related news.

Earlier Monday morning, federal Judge Aileen Cannon dismissed the criminal case charging the former president with illegally retaining classified documents and obstructing the government's efforts to retrieve them.

— CNBC's Josephine Rozzelle contributed reporting.