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President Donald Trump walks to the Oval Office with Sen. Lindsey Graham, R-S.C. File/Evan Vucci/AP

When President Donald Trump sought to crack down on the U.S.-Mexico border, U.S. Sen. Lindsey Graham — who for years pursued moderate compromises on immigration to the chagrin of conservative activists — came forward with his own bill to place tight restrictions on the asylum process.

After breaking with many Republicans to confirm President Barack Obama's Supreme Court nominees, Graham went to bat for Trump's judicial picks, most memorably his emotional outburst during Brett Kavanaugh's Supreme Court nomination hearing.

While businesses back home in South Carolina complain about the impact of Trump's trade war, Graham responded by telling them they've "just got to accept the pain that comes with standing up to China."

And, most recently, in the face of rising Democratic calls for Trump's impeachment, Graham stood firmly by the side of a president whom he once called a "kook" and "unfit for office," dismissing allegations of impropriety in Trump's apparent efforts to get foreign countries to investigate his domestic political rivals.

But when it comes to Middle East foreign policy, the issue Graham has long felt most strongly about, South Carolina's Republican senior senator has never hesitated to publicly condemn Trump each and every time he has sought to retreat from the region.

That recurring conflict reared its head again Monday morning, as Graham castigated Trump's abrupt decision to withdraw U.S. troops from northern Syria as "a disaster in the making."

Graham, along with many fellow lawmakers on both sides of the aisle — plus Trump's own former ambassador to the United Nations, former S.C. Gov. Nikki Haley — fears the move would leave America's Kurdish allies under grave threat from their Turkish neighbors and risk reinvigorating ISIS.

"I like President Trump, I’ve tried to help him," Graham said as he called into "Fox and Friends," well known as the president's favorite morning talk show. "This to me is just unnerving to its core. To say to the American people ISIS has been destroyed in Syria is not true."

Graham's split from Trump on foreign policy is nothing new, even if this particular episode may have elicited some of his harshest criticism yet.

A military hawk with an expansive view of America's role in the world was always destined to clash with a president who campaigned on ending what he termed America's "stupid wars." They now have several times.

Graham said he was "terribly disappointed" with Trump's 2017 decision to try terrorism suspects in criminal court rather than declare them enemy combatants and described a possible peace agreement with the Taliban in Afghanistan — which has since fallen apart — as "a bigger mistake than Obama’s Iranian nuclear deal."

After a March 2018 visit to the region, Graham penned a Wall Street Journal op-ed that said he did not see "a coherent strategy from the administration." When Trump last considered leaving Syria in December 2018, Graham warned of "devastating consequences." 

In a play to Trump's loathing of his predecessor, Graham often invokes Obama comparisons to try to convince Trump to change course — a rhetorical tool he deployed again Monday.

"If I didn’t see Donald Trump’s name on the tweet, I thought it would be Obama’s rationale for getting out of Iraq," he said on Fox and Friends. 

Graham has been forthcoming about the fact that his attempts to build a friendly relationship with Trump on other issues is in part designed so that he can try to influence his foreign policy decisions. Trump has acknowledged that he hears Graham out on foreign policy issues. 

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"I really enjoy the time I have with the president," Graham said in an August interview with The Post and Courier. "I'm in a spot where I can be me. On national security, I'm in a different place than probably he is, but I'm trying to help him where I can."

Graham's stronger reputation among Republicans in South Carolina than in years past may also give him more leeway to openly disagree with a president who remains overwhelmingly popular with the GOP base.

Up for re-election in 2020, Graham has likely avoided the possibility of a credible primary challenger. He is now spending the political capital he raised.

"After the Kavanaugh hearings, he seems to be in much better standing, and he's able to take advantage of it without fear of the political repercussions," said Clemson political science professor Dave Woodard, who managed Graham's first campaign for Congress in 1994.

For all the fire and brimstone, the disputes have yet to land a fatal blow on Graham's relationship with Trump. After previous disagreements, the two often ended up right back on the golf course together just a few days later as if nothing had happened.

Whether this time will be different remains to be seen. But Graham does not appear to be worried.

"I lose zero sleep about saying nice things about him," Graham said in the August interview. "I also lose zero sleep about saying, 'That's nuts.'"

Follow Jamie Lovegrove on Twitter @jslovegrove.

Jamie Lovegrove is a political reporter covering the South Carolina Statehouse, congressional delegation and campaigns. He previously covered Texas politics in Washington for The Dallas Morning News and in Austin for the Texas Tribune.