WASHINGTON (AP) — When Ohio Sen. JD Vance traveled to the Munich Security Conference earlier this year, he wasn't there to reassure Europe and other global allies that America would aid Ukraine in its war against Russia, as all of the other senators were.

Instead, Vance was there to deliver what he called a "wake-up call."

In a fiery speech, the freshman senator said that just because Russian President Vladimir Putin is a "bad guy" doesn't mean that America's own interests shouldn't come first.

"There are a lot of bad guys all over the world, and I'm much more interested in some of the problems in East Asia right now than I am in Europe," Vance said, arguing that the U.S. doesn't have the money or the manufacturing capacity to help Ukraine enough to win the war.

Now elevated into the national spotlight as former President Donald Trump's running mate, Vance's short Senate career reflects the forces transforming the Republican Party. In just a year and a half, he has carved out a unique role fighting traditional Republican establishment figures, including Senate Republican Leader Mitch McConnell, particularly on foreign policy and aid to Ukraine. The 39-year old has been unafraid to challenge his fellow senators, many of them decades older, espousing a populist vision and defending Trump when many of his Republican colleagues stayed quiet.

"He was not afraid to go and speak his opinion to people who didn't necessarily agree with him," said Wyoming Sen. John Barrasso, a senior Republican in the Senate, of Vance's Munich trip. Barrasso, who who has grown close to Vance and advised him when he first came to Washington, said it was a "sign of boldness."

Kentucky Sen. Rand Paul, a Republican who has long opposed aid for foreign conflicts, celebrated Vance's selection. "It's great to have an opponent of endless wars and more aid to Ukraine on the ticket," he wrote on X.

Vance, who served in Iraq as a Marine and is the author of the memoir " Hillbilly Elegy," was once a caustic critic of Trump, but became a fierce ally during his 2022 Senate race, landing Trump's crucial endorsement in the final weeks of a crowded GOP primary.

He then entered the Senate as one of Trump's most vocal supporters, endorsing him in a January 2023 op-ed when many of his colleagues thought the former president's political future was over. Vance argued that a series of foreign conflicts had failed to serve America's interests, and "very few were ever challenged by a leader of national significance. That is, of course, until Donald Trump came along."

Vance's views on America's role in the world put him in conflict with McConnell, despite significant financial help from the McConnell-backed Senate Leadership Fund in the final months of Vance's campaign. As McConnell pushed for the last aid package to Ukraine, approved in April, Vance was its loudest Senate opponent.

"Why are we so obsessed with this? It is a fetish, Steve. I can't pretend to understand it," Vance said on Steve Bannon's podcast in February.

Vance's outspoken views on Ukraine prompted concerns across that region on Tuesday. "If this couple (Trump and Vance) wins, it will not be easy for us. It will not be easy for Europe. It will not be easy for the US," posted Ukraine analyst Yurii Bohdanov on his Telegram channel.

At a press conference in Kyiv, Volodymyr Zelenskyy said that while most of the Democratic Party supports Ukraine, there are varying positions among Republicans, some of whom are "more right-wing and radical."

"If Mr. Donald Trump becomes president, then we will work with him" Zelenskyy said. "I am not afraid of it."

Concerns about what Vance's ascension might mean for U.S. foreign policy extend beyond Ukraine.

European diplomats were loath to comment Tuesday on Vance, citing a desire not to be seen as interfering in American politics. But two U.S. officials who spoke on condition of anonymity to discuss private diplomatic conversations said they had gotten queries from counterparts in Europe asking how the Biden administration's commitment to the continent's security might be preserved if Trump were elected.

Vance has also defended Trump's actions after the 2020 election, when the former president pressured states and Congress to try and overturn President Joe Biden's legitimate victory. Vance denies that Trump tried to overturn the election and said he is "truly skeptical" that former Vice President Mike Pence's life was in danger on Jan. 6, 2021, when Trump's supporters violently besieged the Capitol as Congress certified Biden's victory. Some were chanting "hang Mike Pence" because Pence wouldn't go along with Trump's efforts.

Biden campaign chairwoman Jen O'Malley Dillon said that Trump picked Vance "because Vance will do what 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."

Vance's quick rise to popularity in Trump's inner circle was hardly assured, especially after he'd been critical of then-candidate Trump in 2016.

Donors didn't initially want to talk to Vance as he ran for Senate, said a Republican familiar with the situation and granted anonymity to discuss it. Then wealthy entrepreneur Peter Thiel, a tech donor some see as a mentor to Vance, swooped in with millions for the primary race before the Senate Leadership Fund helped Vance in the general election.

As Vance turned himself into the "soul of MAGA" in the Senate, as the Republican called him, a key turning point for both Trump and Vance was a trip to East Palestine, Ohio, in early 2023 after a train full of toxic chemicals derailed there. As Biden initially stayed away, some Republicans credited the visit with boosting Trump's own flagging campaign.

Vance later allied with Democratic Sen. Sherrod Brown, his Ohio colleague, to introduce legislation that would force railroads to follow new safety rules. The bill has since stalled, opposed by other Republicans, including McConnell, who think it is too favorable to unions and oppose new regulations on industry.

Like the railroad bill, Vance has teamed with Democrats on other issues that reflect his more populist vision. He worked with Massachusetts Sen. Elizabeth Warren, D-Mass., on legislation that would claw back compensation for bank executives in the event of a bank failure, for example, and he was a critic of Boeing after a safety incident on one of their planes earlier this year.

"JD can really speak to this whole group of Democrats who have been left out, left behind by their party," Barrasso said.

Still, Vance is largely untested on the national stage. And while his views may help the Trump ticket among white working-class voters in Pennsylvania, Michigan and Wisconsin, he could potentially turn off others in the swing states of Arizona, Nevada and Georgia.

Republican colleagues say his youth and conviction will ultimately win out.

Alabama Sen. Katie Britt, who was also elected in 2022, says Vance has distinguished himself in the GOP conference because he "is unafraid to enter any room." And as a fellow parent of younger children, "we come at it with a different perspective and a different energy," she said.

"He has a lot of self confidence," says Texas Sen. John Cornyn, whom Vance once worked for as a law clerk on the Senate Judiciary Committee. "JD really represents the next generation of leaders — he's obviously only 39 years old but I think he's prepared for the job."

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Associated Press writers Hanna Arhirova in Kyiv, Ukraine and Matthew Lee in Washington contributed to this report.