The opinions of Mr. Blunt and his colleagues also align with those of much of the Republican foreign policy establishment, current and former top members of the military, and many conservative media commentators, bolstering their willingness to speak out. There truly is strength in numbers. Just a few Republicans — notably Senators Rand Paul of Kentucky and Mike Lee of Utah, both avowed members of the party’s noninterventionist wing — hailed the president’s decision.

“Foreign policy has always been Trump’s Achilles’ heel with Senate Republicans,” said Alex Conant, a Republican strategist and former staff adviser to Senator Marco Rubio of Florida, another Republican who faulted the president’s Syria decision — but not his comments about China and Ukraine. Mr. Conant said Republicans were also driven by their view that Mr. Trump’s foreign policy missteps were more damaging, requiring a more forceful response than his day-to-day incendiary statements.

“Everyone forgets Trump’s tweets after a couple of days,” Mr. Conant said. “But history will never forget if the U.S. allows our Kurdish allies to be massacred.”

At the same time, foreign policy — unlike, say, impeachment — is lower on the president’s priority list. Differences of opinion are less likely to spur him to lash out as he has in recent days, for example, at Senator Mitt Romney, Republican of Utah, after Mr. Romney said that the president’s requests of Ukraine and China to investigate Vice President Joseph R. Biden Jr. were “wrong and appalling.” Mr. Trump responded with name-calling, disparagement and a gleeful reminder of Mr. Obama’s defeat of Mr. Romney in 2012.

Foreign policy appears to be one of the few areas where Mr. Trump is willing to brook some difference of opinion. Pressed on Monday about the tough criticism of his Syria policy from the likes of Mr. Graham; Senator Mitch McConnell, Republican of Kentucky and the majority leader, and Nikki R. Haley, Mr. Trump’s former United Nations ambassador, the president was uncharacteristically restrained.

“I have great respect for all of the people that you named,” Mr. Trump said. “And they have their opinion, and a lot of people do. And I could also name many more than you just named of people that totally are supportive. You see the names coming out; people are extremely thrilled because they say it’s time to bring our people back home.”