Sen. Mitt Romney Willard (Mitt) Mitt RomneyCrenshaw looms large as Democrats look to flip Texas House seat The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election Trump dumbfounds GOP with latest unforced error MORE (R-Utah) on Monday knocked 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's decision to pull back U.S. troops in northern Syria, joining a chorus of high-profile Republicans breaking with the president.

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"The President’s decision to abandon our Kurd allies in the face of an assault by Turkey is a betrayal. It says that America is an unreliable ally; it facilitates ISIS resurgence; and it presages another humanitarian disaster," Romney tweeted.

The President’s decision to abandon our Kurd allies in the face of an assault by Turkey is a betrayal. It says that America is an unreliable ally; it facilitates ISIS resurgence; and it presages another humanitarian disaster. https://t.co/Tu8ARa8Pmp — Senator Mitt Romney (@SenatorRomney) October 7, 2019

The White House announced late Sunday that Ankara was moving forward with a long-threatened offensive in northeastern Syria and that U.S. troops would not be in the “immediate area” when it happens.

Trump doubled down on the decision on Monday, tweeting that it was time to pull the United States out of "ridiculous Endless War."

"WE WILL FIGHT WHERE IT IS TO OUR BENEFIT, AND ONLY FIGHT TO WIN,” the president added.

Around 1,000 U.S. troops have been deployed in northeastern Syria, where they work closely with the Kurdish YPG, which leads the Syrian Democratic Forces in the region.

Trump's move was quickly panned by several GOP lawmakers, including Sens. 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.) and Marco Rubio Marco Antonio RubioGOP lawmakers distance themselves from Trump comments on transfer of power McConnell pushes back on Trump: 'There will be an orderly transition' Graham vows GOP will accept election results after Trump comments MORE (R-Fla.), who serve with Romney on the Senate Foreign Relations Committee.

Republicans say the decision will bolster the Islamic State in Iraq and Syria (ISIS) and put the lives of Kurdish allies in the region at risk.

Romney's public pushback comes after Trump spent the weekend lashing out at the GOP senator, including calling him a "pompous ass" and suggesting he should be impeached.

"I’m hearing that the Great People of Utah are considering their vote for their Pompous Senator, Mitt Romney, to be a big mistake. I agree! He is a fool who is playing right into the hands of the Do Nothing Democrats! #IMPEACHMITTROMNEY," Trump tweeted.

Romney is part of a small group of GOP senators who have publicly pushed back against Trump for saying late last week that China and Ukraine should investigate former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE and his son Hunter Biden.

"When the only American citizen President Trump singles out for China’s investigation is his political opponent in the midst of the Democratic nomination process, it strains credulity to suggest that it is anything other than politically motivated," Romney said in a statement on Friday.

Romney also voiced concerns late last month about the phone call where Trump asked the Ukrainian government to help his personal attorney, Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE, look into the issue. There's been no evidence of wrongdoing by Biden.

"I did read the transcript. It remains troubling in the extreme. It's deeply troubling," Romney told reporters last week. "Clearly what we've seen from the transcript itself is deeply troubling."