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 at a critical moment in his presidency. Democrats are pursuing an impeachment inquiry in the House, while Republicans in the Senate stand as the president's bulwark against being removed from office.
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."
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."
And Senate Majority Leader Mitch McConnell likened Trump's latest foreign policy announcement to something from Barack Obama's presidency. "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.
Republicans in Congress have broken with Trump on Syria before. The GOP-controlled Senate voted overwhelmingly in February to oppose the withdrawal of U.S. troops from Syria and Afghanistan the last time he unveiled a similar proposal without warning on Twitter late last year.
But the intensity, scope and timing of Monday's backlash makes this time different. In the face of a serious impeachment inquiry, Trump's very political future depends on his ability to maintain the loyalty of his party on and off Capitol Hill.
"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," said former Trump aide Alex Conant, 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. For a day, at least, the intraparty clash dominated the political conversation, overshadowing the president's near-constant campaign to undermine the Democrats' impeachment investigation.
Nikki Haley, who was Trump's hand-picked ambassador to the United Nations, 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.
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." 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." 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."
While unusually strong, the Republican rebellion was far from complete. And polling suggests that voters in both parties share Trump's desire to end the nation's decadeslong military operations in the Middle East.
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."
Trump defended his decision among a stream of more than a dozen social media posts Monday, downplaying the risk of a potential Turkish attack on the Kurds. "If Turkey does anything that I, in my great and unmatched wisdom, consider to be off limits, I will totally destroy and obliterate the Economy of Turkey," Trump wrote. He added: "The endless and ridiculous wars are ENDING! We will be focused on the big picture, knowing we can always go back & BLAST!"
Associated Press writers Sagar Meghani and Alan Fram in Washington contributed to this report.