Russia’s invasion of Ukraine is emerging as a serious test for former President Trump and his “America First” approach to the world, as rising public support in the U.S. for the effort to counter Moscow threatens to undermine a key pillar of the former president’s political brand.

Trump’s long-held grievances about NATO and other multinational partnerships — grievances that came to define the modern GOP’s approach to foreign policy — have suddenly put him out of step with many in his party, who have begun calling for the U.S. to take a more active leadership role in the crisis in Ukraine.

At the same time, his praise of Russian President Vladimir Putin has drawn rare bipartisan criticism, with even former Vice President Mike Pence speaking out against “apologists for Putin” within the GOP. Others have also struck a more hawkish tone. Sen. Rick Scott (R-Fla.) said last week, for instance, that deploying U.S. ground forces to Ukraine shouldn’t be taken “off the table.”

The rhetoric marks a rare break between Republicans and a former president who has maintained a vise-like grip over GOP orthodoxy and ideology since first clinching the party’s presidential nomination in 2016. And as he eyes another bid for the White House in 2024, the shifting sentiment on the United States’ role in the world could pose a challenge for Trump.

“You’ve already seen the Republican Party backing away from the former president’s record and his statements as this all unfolded, and trying to reassert the party as more hawkish than they have been over the last four years,” Julian Zelizer, a professor of history and public affairs at Princeton University, said.

To be sure, poll after poll has shown that Americans have little appetite for a war with Russia. It’s also unclear how long Moscow’s military campaign in Ukraine will last and whether it will evolve into a wider conflict. The 2024 Republican primaries are also nearly two years away, and early polling shows Trump as the heavy favorite for the nomination.

Doug Heye, a Republican strategist, said he doesn't think Trump's brand will be hurt with Republicans even as the Russian invasion continues to dominate the headlines.

“One of the consistent things we've seen from Republicans is that they can differ with Trump, even criticize administration policy, as long as they don't cross him on the things that are Trump-centric,” Heye said. “So they'll be critical on Russia or China policy, but backing him — or at least shutting up — on the 2020 election and Jan. 6.”

Still, in one sign of the shifting tides, a Quinnipiac University poll released earlier this week found that an overwhelming majority of Americans — 79 percent — would support a military response if Russia were to attack one of the United States’ NATO allies, including 82 percent of Republicans.

Trump has long criticized the alliance between the U.S., Canada and European nations, complaining that other member countries were not spending enough on the shared costs of defense. At one point during his presidency, Trump reportedly suggested withdrawing the U.S. from NATO.

Another poll from the liberal-leaning firm Navigator Research found that most Americans — 56 percent — disagreed with the notion that the U.S. “should stay out” of the Ukraine crisis in order to “protect American interests and the economy,” an apparent rejection of isolationist sentiment.

Ford O’Connell, a Trump-aligned Republican strategist and former congressional candidate, rejected the notion that the former president’s “America First” doctrine was akin to isolationism, arguing that Trump helped reshape GOP foreign policy in a way that more accurately addressed modern challenges. Republicans, he said, would be wise to stick with the former president.

“Trump redefined it,” O’Connell said, referring to Trump’s influence over Republicans’ foreign policy agenda. “He railed against Republican foreign policy for quite some time and he redefined it into something that better meshes with the realities of the world.”

“When it comes to people like [Chinese President] Xi [Jinping] and Putin, they don’t care about traditional diplomacy in the way, say, the United Nations does,” he added. “I think Trump was very, very wise to that.”

Nevertheless, the spotlight on Russia and Ukraine has renewed focus on roles that both countries played during Trump’s tenure in the White House. In particular, Trump was impeached for the first time in 2020 for trying to pressure Ukrainian President Volodymyr Zelensky to investigate his Democratic opponent, now-President Biden, and Biden’s son Hunter.

Trump also repeatedly praised Putin during his time in the Oval Office, drawing criticism from Democrats who accused him of being too cozy with a U.S. adversary. Even after Russia’s invasion late last month, Trump called the attack “savvy” and “genius” — a position he has since tried to temper.

Still, Dan Eberhart, a GOP donor, said that Trump has yet to convey a clear position on Russia’s invasion of Ukraine. 

And if the crisis remains in the political spotlight through 2024, it could pose a significant challenge for Trump, especially as some his potential rivals for the GOP nomination look to more aggressively tackle the issue. Pence, for instance, traveled to the Ukraine-Poland border on Thursday and met with Ukrainian refugees who had fled the conflict in their home country.

“Trump has softened his position of Putin being a genius, but he hasn’t landed on a coherent position,” Eberhart said. “He’s anti-[European Union], anti-NATO, anti-Biden. He’s not really built for this kind of crisis.”

Democrats are also eager to hit Trump over his past rhetoric on Putin and Ukraine. Michael Trujillo, a Democratic strategist, said that the crisis has effectively exposed a rift between the former president and the party that he has sought to tightly control.

“Donald Trump has learned over time that his position on anything becomes the Republican Party position,” Trujillo said. “However, on Ukraine, he is going to sadly find out that is not the case and that he is losing his grip on his party, the public and it will emerge he is merely a puppet of Vladimir Putin.”

Another Democratic strategist, Christy Setzer, said that her party should proactively seize on Trump’s warm rhetoric toward Putin and use it to draw battle lines well before he ever launches a potential 2024 campaign. Whether that will actually happen is another question, she said.

“We should absolutely, and soon, talk about Trump-Putin Republicans, opposed to the Biden-Zelensky Democrats and make this a 'whose side are you on?' moment,” she said.

“Will we? Probably not yet, and not enough,” Setzer added. “For now, the White House is still leaning into unity and leadership, implicitly showing that it's the GOP who's out of step with the country, and the world. The problem: Implicitly showing anything usually isn't enough.”

The sudden increase in public support for Ukraine and American allies in Europe amid the Russian invasion isn’t entirely unparalleled. Scarred by the aftermath of World War I and, a decade later, the Great Depression, Americans retreated inward before rallying around their European allies as World War II heated up.

Zelizer, the history professor, said that the United States’ involvement in World War II isn’t necessarily an analogue for its current posture toward the situation in Ukraine.

For one, there’s little indication that Americans are eager for a direct military confrontation with Russia. And the advent of nuclear weapons — which didn’t exist in 1941 when the U.S. entered World War II — makes the stakes of a conflict with Russia exponentially higher.

Still, Zelizer said that if anything, Russia’s invasion of Ukraine is a reminder “that real dangers exist out there” and that such international events have the potential to quickly reshape the political landscape within the U.S.

“I think what the invasion has done — even if it has not generated support for military action — it has made clear that alliances matter; that the United States’ role in the world matters,” he said. “It’s showing that real dangers exist out there. You can talk about ‘America First’ all you want but that doesn’t get rid of the national security crises that exist.”