Sen. Mitt Romney (R-Utah) on Thursday harshly criticized President Trump’s decision to withdraw U.S. forces from northern Syria, describing the move as “a bloodstain in the annals of American history.”

Romney’s denunciation of Trump’s decision came hours after Senate Republicans blocked a vote on a House-passed resolution condemning the military move.

His speech and the actions of several conservatives reflected the fracture in the GOP over the issue, with many Republicans opposing Trump’s move and others echoing his frustration with “endless war.”

“The cease-fire does not change the fact that America has abandoned an ally, adding insult to dishonor,” Romney said in a scathing floor speech, shortly after Vice President Pence announced that Turkey had agreed to pause its offensive against the Syrian Kurds for five days. “The administration speaks cavalierly, even flippantly, even as our ally has suffered death and casualty. Their homes have been burned and their families have been torn apart.”

The Utah Republican and 2012 presidential nominee added that “the decision to abandon the Kurds violates one of our most sacred duties.”

“It strikes at American honor,” he said. “What we have done to the Kurds will stand as a bloodstain in the annals of American history.”

Romney did not mention Trump by name. But the speech marked his strongest criticism of the president’s actions to date.

Earlier this month, Romney tweeted that it was “wrong and appalling” of Trump to call on China and Ukraine to launch investigations into one of his political opponents, former vice president and 2020 Democratic presidential contender Joe Biden.

Trump responded by calling Romney a “pompous ‘ass’ ” and calling for his impeachment, even though U.S. senators cannot be impeached.

Democrats and a number of other Republicans also have denounced Trump’s move as an abandonment of U.S. allies: the Syrian Kurdish militias that partnered with the U.S. military to fight the Islamic State militant group, also known as ISIS.

On Wednesday, the House passed a bipartisan resolution condemning Trump’s action on an overwhelming 354-to-60 vote. Senate Minority Leader Charles E. Schumer (D-N.Y.) attempted to force Senate passage of the measure Thursday afternoon.

“He does not grasp the gravity of the situation. He doesn’t understand it,” Schumer said of Trump. “The most important thing we can do right now is send President Trump a message that Congress, the vast majority of Democrats and Republicans, demand to reverse course.”

But several Republican senators, led by Sen. Rand Paul (Ky.), stood up to object, thus blocking passage of the resolution.

“The Constitution is quite clear: No authorization has ever been given for the use of force in Syria, no authorization of declaration of war, no permission to be there at all,” Paul said. “So if they want to insert themselves into the civil war, by all means, let’s have a debate, let’s have the constitutional debate, but I for one am not willing to send one young man, or one young woman, one soldier, over there without a clear mission.”

Also objecting to the resolution were Senate Foreign Relations Committee Chairman James E. Risch (R-Idaho) and Sens. Mike Lee (R-Utah) and Mike Braun (R-Ind.).

Senate Majority Leader Mitch McConnell (R-Ky.) said Thursday that his preference is for legislation even stronger than the House-passed resolution, arguing that the measure had some serious weaknesses.

Among the Republicans criticizing Trump’s decision on Thursday was Sen. Patrick J. Toomey (Pa.), who issued a statement in support of a bipartisan measure that would levy sanctions against Turkey.

“I continue to believe the dangers of withdrawing U.S. forces from northern Syria outweigh the benefits to American security interests,” Toomey said. “Nevertheless, I hope the ceasefire announced today leads to a meaningful and durable outcome that protects the Syrian Kurds and prevents a resurgence of ISIS. This bipartisan bill will strengthen the hand of the United States as we negotiate an acceptable resolution with the Turkish government.”

Trump called Sen. Lindsey O. Graham (R-S.C.), a close ally who had been one of the president’s most vociferous critics on his Syria decision, and told the senator that the agreement was “better than I thought it would be,” in Graham’s retelling.

Graham, who was also briefed by White House national security adviser Robert O’Brien on the deal, said he will continue to build support for sanctions legislation he formally unveiled Thursday with Sen. Chris Van Hollen (D-Md.), arguing that the threat of penalties against Erdogan had successfully put pressure on the Turkish government.

Still, asked how confident he was that the two countries could negotiate a permanent cease-fire after the 120-hour period, Graham responded: “It’s the Mideast, for God’s sake.”

Risch (R-Idaho) is also working on legislation with his counterpart, ranking Democrat Robert E. Menendez (D-N.J.), that would not only impose sanctions against Turkey but also force the administration to present its strategy for the region.

Those bills are two of several punishing Turkey and addressing the ongoing situation that have circulated on Capitol Hill in recent days.

“Whether we go forward with it or not, I think over the next 48 hours we’re gonna get a lot better feel about how serious the Turks are about this and what the reality of the commitments are,” Risch said.

Risch also said multiple senators have received “very specific commitments from the administration that although there was troop movement there, they were not moved such a distance that they would not be able to respond to an ISIS situation in Syria.”