NEW YORK (AP) — They may have his back on impeachment, but some of President Donald Trump’s most loyal allies are suddenly revolting against his decision to pull back U.S. troops from northern Syria.

On Monday, one chief Trump loyalist in Congress called the move “unnerving to the core.” An influential figure in conservative media condemned it as “a disaster.” And Trump’s former top NATO envoy said it was “a big mistake” that would threaten the lives of Kurdish fighters who had fought alongside American troops for years.

Trump’s surprise move, which came with no advance warning late Sunday and stunned many in his own government, threatened to undermine what has been near lockstep support among Republicans. It also came against the backdrop of a congressional impeachment inquiry in which the backing of Republicans in the Senate is the president’s bulwark against being removed from office.

Sen. Lindsey Graham, who has been among Trump’s most vocal defenders, called the Syria decision “a disaster in the making.”

Sen. Lindsey Graham, R-S.C., who has been among Trump’s most vocal defenders, called the Syria decision “a disaster in the making” that would throw the region into chaos and embolden the Islamic State group.

“I hope I’m making myself clear how short-sighted and irresponsible this decision is,” Graham told Fox News. “I like President Trump. I’ve tried to help him. This, to me, is just unnerving to its core.”

Sen. Marco Rubio, R-Fla., who has shrugged off the key allegation in the impeachment inquiry — that Trump pressured foreign powers to investigate a top Democratic rival — tweeted that Trump’s shift on Syria is “a grave mistake that will have implications far beyond Syria.”

WATCH: Rubio: Trump ‘about to make a major blunder’ on Syria

And Sen. Susan Collins, R-Maine, who has been more willing than many Republicans to condemn Trump’s calls for foreign intervention in the 2020 election, called the Syria move “a terribly unwise decision” that would “abandon our Kurdish allies, who have been our major partner in the fight against the Islamic State.”

A more frequent Republican Trump critic, Utah Sen. Mitt Romney, cast Trump’s announcement as “a betrayal.”

“It says that America is an unreliable ally; it facilitates ISIS resurgence; and it presages another humanitarian disaster,” Romney tweeted.

Nikki Haley, who was Trump’s hand-picked ambassador to the United Nations, also cast the decision to withdraw U.S. troops from northern Iraq as a betrayal of a key ally.

“The Kurds were instrumental in our successful fight against ISIS in Syria. Leaving them to die is a big mistake,” she wrote on Twitter.

Former Rubio aide Alex Conant highlighted the risks ahead for a president whose political future depends on Republican support.

“For Trump to make a very controversial move on Syria at the exact moment when he needs Senate Republicans more than ever is risky politics,” Conant said, noting the significance for many Senate Republicans of the United States’ policy in northern Syria, where Kurds would be particularly vulnerable to a Turkish invasion.

“They’re not just going to send out a couple of tweets and move on,” Conant said. “At the same time, the White House is going to need these guys to carry a lot of water for them.”

While a number of Republicans criticized Trump’s decision, one of their most important leaders, Senate Majority Leader Mitch McConnell of Kentucky, was sanguine, offering little concern about Syria or impeachment during an appearance at the University of Kentucky.

“There are a few distractions, as you may have noticed,” McConnell said. “But if you sort of keep your head on straight and remember why you were sent there, there are opportunities to do important things for the country and for the states that we represent.”

After the appearance, McConnell issued a statement warning that Trump’s proposed withdrawal “would only benefit Russia, Iran, and the Assad regime. And it would increase the risk that ISIS and other terrorist groups regroup.”

“As we learned the hard way during the Obama Administration, American interests are best served by American leadership, not by retreat or withdrawal,” McConnell said.

Outside government, leaders of conservative groups backed Trump.

Liberty University President Jerry Falwell Jr., a prominent evangelical leader, said Trump was simply “keeping his promise to keep America out of endless wars.”

He suggested Trump could easily reengage in the region if the decision backfires.

“The president has got to do what’s best for the country, whether it helps him with this phony impeachment inquiry or not,” Falwell said in an interview.

Former Trump campaign aide Barry Bennett noted that the president has been talking about reducing troop levels in the Middle East since before the 2016 election.

“I understand that they don’t like the policy, but none of them should be shocked by the policy,” Bennett said. “He’s only been talking about this for four or five years now. I think he’s with the vast majority of the public.”

Still, the backlash from other Trump loyalists was intense.

Rep. Elise Stefanik, R-N.Y., a member of the House Armed Services and Intelligence committees, called it a “misguided and catastrophic blow to our national security interests.”

And on Fox News, a network where many rank-and-file Trump supporters get their news, host Brian Kilmeade said it was “a disaster.”

“Abandon our allies? That’s a campaign promise? Abandon the people that got the caliphate destroyed?” Kilmeade said on “Fox & Friends.”

Bulent Aliriza, director of the Turkey Project at the Center for Strategic and International Studies, said the controversy reminds him of former Defense Secretary James Mattis’ decision to resign late last year after Trump announced plans to withdraw troops from Syria.

“Ultimately, Trump reversed himself,” Aliriza said. “The question is whether he will actually reverse himself again in view of the opposition from Capitol Hill led by several of his closest allies.”

Associated Press writers Sagar Meghani and Alan Fram in Washington contributed to this report.