In the closing months of a crowded Republican primary for a US Senate seat in Ohio, JD Vance found himself stuck in the middle of the pack.

He appeared badly damaged by a barrage of ads painting the venture capitalist and former Donald Trump critic as an anti-MAGA San Francisco liberal. The pollster for a supportive super PAC warned that Vance’s campaign was in “precipitous decline,” arguing that he had failed to convince Republican voters of his conservative bona fides and loyalty to the former president.

“Vance needs a course correction ASAP,” the pollster wrote in a February 2022 memo.

It arrived a month later. With the five main primary contenders meeting onstage for the umpteenth time, the two perceived front-runners nearly came to blows. As they stood nose to nose, one readied to fight while the other uttered a sexist expletive. Vance, seated at the edge of the stage, pounced.

“Think about what you just saw. This guy wants to be a US senator and he’s up here, ‘Hold me back. Hold me back,’” Vance said to loud applause. “What a joke. Answer the question. Stop playing around.”

It was a breakthrough moment for Vance, one that led to a second look from GOP voters in his state and from Trump, who was closely watching the race but hadn’t acquiesced to the voices in his party urging him to get involved. Clips of the exchange and other debate moments impressed Trump, sources told CNN, and played a role in Vance securing a race-defining endorsement from the former president.

“It was a big moment for the campaign,” said a person involved with Vance’s successful 2022 Senate bid, suggesting that the episode demonstrated his knack for seizing make-or-break opportunities on a debate stage.

On Tuesday, that ability will be tested once again. Vance, now the Republican nominee for vice president, will join his Democratic counterpart, Minnesota Gov. Tim Walz, for the first time on a debate stage in New York.

Vance, at just 40 years old and two years into his political career, is still a largely unproven commodity. Also unknown is whether he can successfully pick apart the Democratic ticket while improving — or at least not further jeopardizing — his likability among voters. And then there’s Vance’s other audience of one, Trump, who often has his own benchmarks for a successful on-air performance.

The stakes of Vance’s head-to-head meeting with Walz are uncharacteristically high for an undercard showdown and illustrative of the exceedingly narrow battle for the White House. Not only is Tuesday’s affair the campaign’s lone vice presidential debate, but it is also likely the last time voters will see the two tickets side-by-side on national television. Trump has suggested it is too late for another debate with Kamala Harris, including one proposed by CNN that the vice president agreed to, and there aren’t any more scheduled between Vance and Walz, either.

(On this, Vance has broken from Trump and called for additional debates with Walz, arguing that “you should have to earn this job.”)

Vance has spent the past month engaging in intense prep sessions, including a mock debate this week with House Majority Whip Tom Emmer playing the role of Walz. On Tuesday, the Ohio Republican downplayed those efforts, asserting that the campaign’s policies speak for themselves.

“What we’re going to focus on is making sure that I make as concise and direct an appeal to the American people as possible about Donald Trump’s successful policies and Kamala Harris’s failed policies,” Vance said.

Vance was chosen by Trump in part for this day. In the weeks leading up to Trump’s running mate decision, the former president regularly commented to those around him how well Vance performed on television. It’s a skill that matters deeply to Trump, one Vance first picked up in the Marines when he was assigned to the public affairs office. There, he learned to “speak clearly and confidently with TV cameras shoved in my face,” he wrote in his memoir “Hillbilly Elegy.”

Between touring to promote his book and launching a political career, Vance honed those capabilities. Some of Trump’s allies tried to sway his vice presidential selection by showing him clips of Vance’s television interviews, sources told CNN. After Trump selected the Ohio senator, advisers told CNN that they were especially looking forward to a Harris-Vance debate, insisting his previous tape matched up favorably against the vice president’s.

That, of course, did not come to be. Instead, when President Joe Biden dropped out of the race and Harris stepped in, Vance lost his opponent, a change in circumstances that, he joked, left him “pissed off.”

“I was told I was gonna get to be Kamala Harris and now President Trump’s gonna get to debate her?” he quipped in July.

Vance’s role leads to combative interviews

Vance enters the debate against Walz in the final weeks of a tight race and following an uneven introduction to Americans, marred by resurfaced clips of him denigrating childless adults and pushing strident anti-abortion views. When Vance was selected as Trump’s running mate in July, his favorability rating was negative 6 points, according to a CNN poll, with nearly 4 in 10 registered voters saying they were unsure about the Ohio Republican. Since then, views of Vance have become more defined, with a CNN survey last week showing his net favorability getting worse, now 12 points underwater, while roughly a quarter of registered voters said they were unsure how they felt about him.

Trump and his advisers insist they are unbothered by America’s view of Vance, asserting that it’s partly a function of the role he has been assigned. The campaign has thrust Vance into less-than-friendly territory, tasking him to defend Trump on air in interviews that are often combative while also leveling increasingly antagonistic shots at the Harris-Walz ticket.

“He’s an attack dog,” one senior adviser said. “That’s part of why he was brought on, and it’s what he does well.”

Since selecting Vance, Trump has asked allies off and on how they think he is doing, a source familiar told CNN. The senior adviser, however, brushed off the idea that the question meant anything about Trump’s opinion of his running mate.

“He does that to everyone. It’s just how he talks,” the adviser said, joking that Trump had asked about the adviser’s performance as well.

Vance’s regular clashes with reporters have won him commendations from conservative pundits and online personalities, but it’s unclear whether that approach will yield dividends in front of the more general audience who will be tuning in for Tuesday’s debate.

Advisers, though, say those encounters have sharpened his retorts, as well as his command of the issues. While Harris and Walz have approached the press with extreme caution, Vance has made it a point to appear accessible.

Since being selected as Trump’s running mate, Vance has participated in dozens of interviews on network television from “Meet the Press” to Fox News to CNN to CBS’ “Face the Nation,” as well as lengthier sit-downs with Sirius XM’s “Megyn Kelly Show” and far-right personality Tucker Carlson. At his campaign events — of which there have been about 30 so far — Vance regularly fields press questions and will gab openly with reporters traveling on his campaign plane between events.

“I have to believe that if you want to be the American people’s president, you ought not be afraid of friendly American media,” Vance said at a recent event in Raleigh, North Carolina.

Often, Vance takes questions with a friendly crowd behind him, who will boo reporters depending on their network or line of questioning and cheer on the senator’s answers — a home-field advantage he won’t have inside the CBS’ New York City studio Tuesday. The debate will not have a live audience.

Vance’s daily practice talking to reporters on the trail has freed him to study up on Walz, who his advisers see as a skilled orator in his own right who shouldn’t be underestimated. The senator’s team has met in person at Vance’s Cincinnati home, as well as on Zoom over the past several weeks, with a focus on helping him better understand Walz stylistically, as well as on familiarizing Vance with Walz’s record as both a Minnesota congressman and governor, CNN previously reported.

On the campaign trail, though, Vance has largely moved on from attacking Walz — whom he needled regularly after the Minnesota governor became the Democratic vice presidential nominee — and instead has focused his attacks more on Harris at recent events. He suggested he intends to take a similar approach Tuesday.

“I’m going to use my debate opportunity to try to prosecute the case against Kamala Harris,” Vance told CNBC earlier this month, “because she’s ultimately going to be the president if the American people elect her.”

CNN’s Alayna Treene contributed to this report.