WASHINGTON — Despite reports that President Donald Trump indirectly threatened to leave NATO over defense spending, a key Republican senator said Thursday that Congress would never let a U.S. president exit the alliance.

“If they were seriously planning on withdrawing from NATO, I think it would create a unifying event unlike anything you’ve seen in U.S. history,” said Sen. Thom Tillis, R-N.C., a leader of the Senate NATO Observer Group and member of the Senate Armed Services Committee.

Congress has multiple ways to stop a withdrawal, and the will is there, evidenced by the Senate’s 97-2 vote to reaffirm support for NATO during the summit last week, Tillis said.

Tillis called such a scenario hypothetical.

Trump has been pressuring member nations to increase their defense spending and has said he could “probably" leave the alliance without congressional approval. The question has distracted from NATO’s more important work, Tillis said.

Tillis and Sen. Jeanne Shaheen, D-N.H., lead the Senate NATO Observer Group. It added 10 senators Tuesday, the day after Trump denied Russia meddled in the 2016 U.S. elections during a news conference with Russian President Vladimir Putin in Helsinki, Finland.

Shaheen, a member of the Senate Armed Services and Senate Foreign Relations committees, said that while Congress and the American public would not allow Trump to exit NATO, Trump’s equivocations on its value and hostility toward allies may eventually persuade allies to go their own way.

“There are only so many times that can happen, and it begins to chip away at the belief the United States is a reliable ally,” Shaheen said. “We need to do everything we can to reassure allies we are a reliable partner.”

Their remarks, at a Center for Strategic and International Studies event on Thursday, came after multiple reports that Trump threatened during the NATO summit in Brussels last week to pull the U.S. out of the alliance if allies did not commit to a defense spending hike. French President Emmanuel Macron has since disputed those reports.

In a Fox News interview released Tuesday, Trump questioned the mutual defense agreement at the heart of NATO — and why any American would have to defend Montenegro. (Montenegro joined NATO in 2017, a year after a Russian-backed plot to overthrow Montenegro’s Cabinet and assassinate its prime minister.)

Trump suggested the country, of fewer than a million people, could somehow provoke a global conflict.

"They're very strong people, they're very aggressive people. They may get aggressive and, congratulations, you're in World War III,” Trump said.

On Thursday, Tillis offered a strong rebuke, noting that more than 1,000 allied troops have died in the wars since the United States invoked the Article 5 mutual defense clause after 9/11.

“These nations that are on the front line. To suggest that they would want to instigate a war would mean that they would want to instigate their own suicide,” Tillis said. “It would make no sense to me that a nation of that size would start the spark that would start a war. I’ve only been [in the Senate] three and a half years. Maybe there’s a logic that I can’t quite get.”

Tillis and Shaheen reestablished the Senate NATO Observer Group earlier this year to consider new accessions to the alliance, address Russian cyberthreats and reaffirm support for the alliance. They attended the NATO summit last week.

The group was founded by then-Sen. Trent Lott in the late 1990s to shepherd NATO enlargement measures through the Senate, and both Tillis and Shaheen voiced support for the accessions of prospective members, like Macedonia.