(Bloomberg) -- The fate of an ethnic minority 6,000 miles away has tested Republican support for Donald Trump more than any of the president’s scandals and allegations of wrongdoing -- right when he needs GOP lawmakers to protect him from impeachment.

Kurdish fighters in Syria had been the closest U.S. allies in the fight against the Islamic State. Trump’s abrupt decision to withdraw U.S. troops from the region opened the door to a Turkish invasion of territory controlled by the Kurds.

The move infuriated lawmakers from both parties and Trump’s outside supporters, especially those with a military background who understand the strategic value of the Kurdish alliance, along with evangelicals who fear for Kurdish Christians.

“I’m heartbroken,” Illinois Representative John Shimkus said Thursday. “I called my chief of staff in D.C., I said pull my name off the ‘I-support-Donald-Trump list.’”

Republicans have for years put up with Trump’s controversial remarks and actions on matters such as porn star Stormy Daniels, neo-Nazi demonstrators in Virginia, and crude comments about women.

The withdrawal from Syria is different because it cuts to the core of U.S. national security, undoing two decades’ worth of efforts to gain the trust of the Kurds and have them fight in the front lines of counter-terrorism efforts so more U.S. troops don’t have to.

Shimkus, a West Point graduate and Army veteran who said he won’t seek re-election, represents a district that Trump won in 2016, and he has generally supported the president. But in a radio interview Thursday, he called Trump’s withdrawal “despicable.”

“This has just shocked, embarrassed and angered me,” he said. The U.S. has “stabbed our allies in the back.”

Wyoming Representative Liz Cheney, a member of GOP leadership, called the withdrawal a “catastrophic mistake.”

Representative Adam Kinzinger, another Illinois Republican and an Air Force veteran, called Trump’s decision “a failure” for abandoning the Kurds and said “the America I love is capable of a much higher calling.”

The cascade of criticism from Republicans could hardly come at a worse time for Trump, who is fending off an impeachment inquiry in the Democratic-led House. That probe is focused on Trump pressuring Ukraine to investigate a domestic political rival.

Shimkus’s communications director, Jordan Haverly, said his comments have “nothing to do with impeachment.”

If the House votes to impeach Trump, it will take a two-thirds majority in the Senate to remove him from office. That makes Senate Republicans, especially Majority Leader Mitch McConnell, the most crucial group to keep in his corner.

McConnell criticized the troop withdrawal on Monday and urged Trump to “exercise American leadership to keep together our multinational coalition” in Syria. McConnell recalled the bipartisan Senate vote in January -- which passed with a veto-proof majority -- to support U.S. troops in northern Syria in response to the last time Trump tried to draw down the U.S. presence in the region.

Senator Lindsey Graham, normally one of the president’s staunchest allies, and Democratic Senator Chris Van Hollen, on Wednesday presented the framework of sanctions on Turkey to deter the incursion that has already begun driving Kurds from northern Syria. Cheney said Thursday that she and almost 30 of her GOP colleagues in the House will introduce a similar measure.

Graham on Thursday said Trump should back the sanctions package to get leverage against Turkish President Recep Tayyip Erdogan.

“It is important that we crush Erdogan before he crushes the Kurds,” Graham said on Family Research Council President Tony Perkins’s radio show.

The Republican senator criticized Trump for not pushing back when he spoke with Erdogan last weekend.

“We backed down. What President Trump should have done when he was called last Sunday, was to tell Erdogan, ‘If you cross into Syria and come after the people who helped us destroy ISIS I will blow you out of the sky,’” Graham said.

Graham said it is “ironic, sad, insulting” that Erdogan is using American-designed F-16s to bomb civilians and Kurdish allies.

A handful of Republicans, including Senators Rand Paul and Mike Lee and Representatives Matt Gaetz and Thomas Massie, have publicly supported Trump’s troop pullback.

(Adds Cheney to back House legislation to sanction Turkey in 15th paragraph)

--With assistance from Emily Wilkins.

To contact the reporter on this story: Daniel Flatley in Washington at dflatley1@bloomberg.net

To contact the editors responsible for this story: Joe Sobczyk at jsobczyk@bloomberg.net, Anna Edgerton, Laurie Asséo

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