Euronews | Adam Edelman with NBC News Politics: A long list of Republicans — including several top allies of President Donald Trump — lined up in vehement opposition on Monday to the president's decision to withdraw U.S. forces from the northern border of Syria and allow a Turkish operation there.

The announcement marked a major blow to the Kurdish-led Syrian Democratic Forces, or SDF, which the U.S. relied upon heavily as the most effective fighting force against ISIS in Syria — and prompted outrage among GOP lawmakers.

In a lengthy tweetstorm, Sen. Lindsey Graham, R-S.C., a fierce Trump loyalist, tore into the decision, calling it a "disaster in the making."

Graham predicted the move "Ensures ISIS comeback," "Forces Kurds to align with Assad and Iran" and "Destroys Turkey's relationship with U.S. Congress," and called it a "stain on America's honor for abandoning the Kurds."

"I feel very bad for the Americans and allies who have sacrificed to destroy the ISIS Caliphate because this decision virtually reassures the reemergence of ISIS. So sad. So dangerous," Graham tweeted. "President Trump may be tired of fighting radical Islam. They are NOT tired of fighting us," he added.

Nikki Haley, who served as Trump's ambassador to the United Nations for nearly two years, tweeted, "We must always have the backs of our allies, if we expect them to have our back. The Kurds were instrumental in our successful fight against ISIS in Syria."

"Leaving them to die is a big mistake," she added, before including a "#TurkeyIsNotOurFriend" hashtag.

Sen. Marco Rubio, R-Fla., said in a Twitter post, "If reports about US retreat in #Syria are accurate, the Trump administration has made a grave mistake that will have implications far beyond Syria."

"It would confirm #Iran's view of this administration & embolden then to escalate hostile attacks which in turn could trigger much broader & more dangerous regional war," he added.

Former Arkansas Gov. Mike Huckabee, another strong supporter of the president, tweeted that abandoning the Kurds was a "HUGE mistake."

"They've never asked us to do THEIR fighting-just give them tools to defend themselves. They have been faithful allies. We CANNOT abandon them," Huckabee wrote.

Sen. Mitt Romney, R-Utah, a Trump critic, weighed in, too, tweeting that the move was a "betrayal."

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

The wave of criticism comes as the U.S. military has moved its forces away from the Turkish border with northern Syria, and after the White House said Sunday night that Turkey would soon begin an operation in a part of northeastern Syria where it wants to resettle Syrian refugees — and that U.S. forces wouldn't be involved.

Analysts and Kurdish-led forces said Monday that the decision risks furtherdestabilizing the war-torn region, undermining the fight against ISIS and sparking a wider conflict between Turkey and Kurdish fighters.

Syrian Kurds — whom Turkey sees as a threat — currently control much of the area close to the border with Turkey, where Ankara hopes to create a so-called safe zone. The Turkish government has long considered Kurdish fighters in Syria as a threat linked to the PKK, a Kurdish group in Turkey that has waged a decades-long insurgency against the government and is considered a terrorist group by the United States. But the U.S. has relied on the Kurdish fighters who lead the Syrian Democratic Forces (SDF) as its most effective partner in the fight against ISIS in Syria.

Trump, however, has remained defiant on the matter, tweeting Monday: "The Kurds fought with us, but were paid massive amounts of money and equipment to do so. They have been fighting Turkey for decades. I held off this fight for almost three years, but it is time for us to get out of these ridiculous endless wars, many of them tribal, and bring our soldiers home."

"WE WILL FIGHT WHERE IT IS TO OUR BENEFIT, AND ONLY FIGHT TO WIN," he wrote.