"He's making the biggest mistake of his presidency," the South Carolina Republican tells reporters in the halls of Congress.

President Donald Trump is annoyed.

"I think Lindsey should focus right now on Judiciary -- like the Democrats -- the do-nothing Democrats," he tells reporters in the East Room of the White House.

Graham is warning about terror attacks.

"He will have American blood on his hands if he abandons Kurds because ISIS will come back."

Trump doesn't want to hear it.

"Lindsey should focus on the do-nothing Democrats."

Watching them launch sharp barbs at one another up and down Pennsylvania Avenue, it's hard to believe Graham is one of Trump's most loyal allies. It's like a time warp -- back to their campaign rivalry.

"I don't think he has the temperament or judgment to be commander in chief," Graham told me in 2016, as he announced he was so disgusted with Trump, he was not going to vote for him for president.

As it turns out, 2016 Graham warned voters about exactly what 2019 Graham is apoplectic about now: that Trump's promise to withdraw troops from the Middle East will make America less safe.

"For God's sakes, pick somebody who is worthy of the sacrifice of those who are fighting this war and who actually knows how to win, and I don't believe that's Mr. Trump," Graham begged voters during a 2016 campaign primary debate.

It's complicated

JUST WATCHED Trump suggests how Sen. Graham could be spending his time Replay More Videos ... MUST WATCH Trump suggests how Sen. Graham could be spending his time 01:23

For Graham and Trump, it's complicated. Very complicated.

On abandoning the Kurds, US allies against ISIS who helped defeat the caliphate, Graham says he is holding Trump to the Barack Obama standard.

"By assuming the Kurds are better off today than they were yesterday -- that is just unbelievable. I can imagine if Obama said that what Republicans would be saying now. So, I'm going to say it with Trump, that is just unfair, dangerous."

But what if Obama called a foreign leader and asked for dirt on his political rival, like Trump did with Ukraine's President on Joe Biden?

It's hard to imagine Graham would say it's no big deal, like he is now.

In fact, Graham is staunchly defending Trump over the Ukraine scandal , even using his powerful role as Senate Judiciary Chairman to backstop the President against House Democrats' impeachment inquiry.

"I can do two things at once," Graham insists.

But that's not how the President sees it.

Trump seems almost disoriented that Graham is not singularly focused on settling scores against Democrats.

"What happened with President Obama? What happened with (Former CIA Director John) Brennan? That's what Lindsey oughta focus on. That's what the people of South Carolina want him to focus on," insisted Trump.

"It's not about me and him," says Graham.

But it is about the two of them and their schizophrenic relationship. The last year and a half of Graham's political life has been defined by a metamorphosis from a maverick John McCain Republican who eagerly bucks the party to a Trump Republican who toes the party line.

It has largely cost him the admiration he reveled in from many Democrats, and among formerly like-minded Republicans, as someone who sees Trump as a danger to the GOP and the country at large.

But Graham had seemed to make peace with that -- arguing privately to friends that the GOP has changed, especially in his home state of South Carolina -- and he decided to change with it. Trump has so much support back home, a top aide calls South Carolina "Trumpistan."

As for Trump, he treasures Graham as a golf buddy who cracks salty jokes and a navigator in the ways of Washington.

And Graham publicly admits that having the President's ear has made him relevant, which he says is important in his line of work.

Influence has its limits

JUST WATCHED Lindsey Graham criticizes Trump over Syria situation Replay More Videos ... MUST WATCH Lindsey Graham criticizes Trump over Syria situation 02:18

Graham has also explained his decision to stay close to Trump as a way to influence him on big policy decisions -- especially ones Graham learned from McCain to care most about: intervening to support allies, especially in the fight against mutual threats.

CNN is told that Graham believes he convinced the President to hold off on making the decision to pull troops from Syria months ago.

Still, he went ahead with it, and their clash is proof that Graham's influence -- his attempts to curb the President's isolationist worldview -- has limits.

"Lindsey Graham would like to stay in the Middle East for the next thousand years with thousands of soldiers fighting other people's wars. I want to get out of the Middle East," Trump said Wednesday.

With that, the President summed up a fundamental difference between the two men that has not changed since 2016, and likely never will.