Sen. Lindsey Graham Lindsey Olin GrahamLincoln Project mocks Lindsey Graham's fundraising lag with Sarah McLachlan-themed video The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election Trump dumbfounds GOP with latest unforced error MORE (R-S.C.) on Wednesday warned that unless President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE reverses course on his decision to pull U.S. troops out of northern Syria, it "will be the biggest mistake of his presidency."

"I hope he's right. I don't think so. I know that every military person has told him don't do this,” Graham said in an appearance on Fox News's “Fox & Friends.”

“So if he follows through with this, it'd be the biggest mistake of his presidency,” Graham continued.

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The often-vocal Trump ally added that "if we pull out, the Kurds are in a world of hurt, and ISIS comes back and President Trump will own it."

Graham's remarks came shortly before Turkish President Recep Tayyip Erdoğan announced that the country's military and a Syrian militia have begun a military operation against Kurdish forces in northern Syria.

Ankara had long threatened to attack the Kurds in Syria but had held off while U.S. troops acted as a buffer. The White House announced on Sunday night that it was yanking U.S. troops from the area.

Trump's decision has faced fierce backlash from both sides of the aisle on Capitol Hill, with lawmakers warning that it will bolster the Islamic State in Iraq and Syria (ISIS) and leave the Kurds vulnerable.

In a string of tweets after the start of the Turkish military operation, Graham said that, if accurate, the move was a "disaster in the making" and that the administration had "shamelessly abandoned" the Kurds, who led the Syrian Democratic Forces that the United States relied on to help fight ISIS.

"If media reports are accurate and Turkey has entered northern Syria — a disaster is in the making," he said. "Pray for our Kurdish allies who have been shamelessly abandoned by the Trump Administration."

Senate Republicans have publicly urged Trump to change his mind about the troop withdrawal. Trump had previously threatened to remove U.S. troops from Syria late last year but ultimately backed down after pressure from the Pentagon and Capitol Hill.

"I urge President Trump to change course while there is still time by going back to the safe zone concept that was working," Graham added on Wednesday.

Lawmakers are warning that they could push back legislatively on Trump's decision. Graham is threatening to slap sanctions on Turkey and author a resolution opposing the president's decision.

Senate Minority Leader Charles Schumer Chuck SchumerPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' 3 reasons why Biden is misreading the politics of court packing Cruz blocks amended resolution honoring Ginsburg over language about her dying wish MORE (D-N.Y) told a New York publication this week that he anticipated legislation given the pushback.

"It is getting broad condemnation from Democrats and Republicans and I expect Congress will take some form of action, either a resolution or something else," he said.