Some of President Trump's strongest supporters in the Republican Party, including South Carolina Senator Lindsey Graham, are condemning his decision to remove U.S. forces from outposts in northeast Syria in order to clear the way for a Turkish incursion. Graham even threatened U.S. action against Turkey if it invades Syria and attacks the Kurdish forces who helped the U.S. defeat the Islamic State in Iraq and Syria (ISIS).

The White House announcement late Sunday that U.S. troops "will no longer be in the immediate area" and the subsequent departure of troops from observations posts at the Syria-Turkey border is considered by some lawmakers to be a betrayal of the Kurds who have fought for years with U.S. forces to fight ISIS.

Graham, who is one of the president's closest allies, warned that should the Turkish carry out an invasion of Syria, Congress would impose bipartisan sanctions that he expects to be "veto-proof," and he said he has already spoken to Democratic Senator Chris Van Hollen about the sanctions. If they attack Kurdish allies, Graham said he would call for Turkey's suspension from NATO.

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He said in a series of tweets, "By abandoning the Kurds we have sent the most dangerous signal possible – America is an unreliable ally and it's just a matter of time before China, Russia, Iran, and North Korea act out in dangerous ways."

He added that he feels "very bad" for U.S. personnel and allies who have made sacrifices to destroy ISIS "because this decision virtually reassures the reemergence of ISIS."

The president, he said, "may be tired of fighting radical Islam. They are NOT tired of fighting us." He warned that the "decision to abandon our Kurdish allies and turn Syria over to Russia, Iran, & Turkey will put every radical Islamist on steroid." It's a "shot in the arm to the bad guys" and "[d]evastating for the good guys."

Former Ambassador to the United Nations Nikki Haley also warned, "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. #TurkeyIsNotOurFriend"

Senate Majority Leader Mitch McConnell issued a statement that said that a "precipitous withdrawal" posed the danger of allowing ISIS to "regroup," and he argued that a new conflict between Turkey and "our partners in Syria" would risk Turkey's ties to the U.S. and cause it to be more isolated.

Florida Senator Marco Rubio tweeted, "If reports about US retreat in #Syria are accurate, the Trump administration has made a grave mistake that will have implications far beyond Syria." He noted that it would "confirm #Iran's view of this administration," potentially causing an escalation in their attacks and possibly a broader regional war.

Maine Senator Susan Collins called it a "terribly unwise decision by the president."

Utah Senator Mitt Romney, who is one of the few Republicans to criticize the president on other issues, also tweeted that the pullback is a "betrayal" that "says America is an unreliable ally." The move, he said, "facilitates ISIS resurgence" and "presages another humanitarian disaster."

House Minority Leader Kevin McCarthy was a little more muted but said on "Fox and Friends" that "if you make a commitment, and somebody is fighting with you, America should keep their word." He also said he would speak to the president. Host Brian Kilmeade urged McCarthy, "Call the president before it's too late."

President Trump defended his decision at length on Twitter, arguing, "the Kurds fought with us, but were paid massive amounts of money and equipment to do so," and their fight with Turkey had been going on "for decades."

The fight against ISIS, he went on to suggest, is a regional problem. "Turkey, Europe, Syria, Iran, Iraq, Russia and the Kurds will now have to...figure the situation out, and what they want to do with the captured ISIS fighters in their 'neighborhood.'" He wrote, "We are 7000 miles away and will crush ISIS again if they come anywhere near us!"

He added in another tweet that "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 (I've done before!)."