Sen. John McCain listens as Sen. Lindsey Graham defends McCain’s military record after Donald Trump’s criticism. (AP Photo/Kevin Hagen, File)

Sen. Lindsey Graham paused for five full seconds and stumbled over his words pondering the question: When is the last time he split with fellow Republican Sen. John McCain on a major issue?

“I don’t know, let me think about it,” Graham (S.C.) finally said of his closest Senate friend. “There have been several. I just can’t recall right now, right off the top of my head.”

Yet that’s what has happened in the wake of Donald Trump’s ascendancy to presumptive Republican nominee for president. In the Republican civil war over Trump, this is perhaps the most glaring example of two “brothers” fighting on opposite sides of the battlefield. It reflects a larger chasm in the Republican Party over whether to embrace the anti-establishment businessman that could end up costing the party the presidency in November.

A former Trump rival in the presidential campaign, Graham is part of the anti-Trump coalition promising to never support the businessman — he has declared the presumptive GOP standard bearer’s positions anathema to conservatives on everything from immigration to fitness to oversee the world’s most powerful military. He told reporters Tuesday that “no re-education camp” would change his mind and added he would likely write someone else in for president when he casts his ballot this fall.

Graham, however, got a surprise call on Wednesday afternoon from the businessman, who was visiting Capitol Hill. They chatted for 15 minutes and agreed to stop insulting one another, but Graham didn’t offer an endorsement.

[Trump and Graham agree to stop the insults]

McCain (Ariz.) is part of the growing ranks of Republicans who, grudgingly, have decided that the voters have spoken and it’s time to unify so they can defeat the likely Democratic nominee, Hillary Clinton.

“I feel, as a Republican and a longtime Reagan Republican, that I support the nominee of the party, and that’s what I’ve said all along,” McCain said.

To be sure, this duo are still close friends. “My dearest friend,” McCain said of Graham in a brief Wednesday interview. “We discuss everything.”

[Graham backs Cruz, calling him best alternative to Trump]

Part of what they discuss is their own political ambition. Graham, re-elected to a term that lasts until 2020, withdrew from a presidential bid before any ballots were cast. McCain must face Arizona voters this fall in his bid for a seventh term.

For more than a decade, the two Republicans have bonded over a shared worldview of a muscular foreign policy. McCain’s outlook was formed as the son of an admiral in a family with military roots that reach to the Revolutionary War, punctuated by his tenure as a Navy pilot and his five-and-a-half years as a prisoner of war in Vietnam. Graham’s conservative outlook was forged out of his southern state’s long military tradition, tinged with a legal view from his years as an Air Force JAG officer.

When the military plan for the Iraq war floundered a decade ago, McCain, 79, and Graham, 60, pushed for the “surge” that turned around the battlefield performance. Graham, along with then-Sen. Joseph I. Lieberman (I-Conn.), were inseparable from McCain during his 2008 presidential bid, earning the “Three Amigos” nickname.

They were so tight, in fact, that McCain used to issue statements about national security along with Graham and Lieberman, as if they were their own special committee of three. When Graham launched his own long-shot bid for the 2016 presidential nomination, McCain stood loyally at his side until the South Carolinian bowed out late last year.

They’ve lost track of how often they traveled together to overseas hotspots. “Oh, more than you can count,” Graham said Wednesday.

Quite simply, there’s no set of senators closer than McCain and Graham. Yet they now disagree on the defining issue of the Republican Party in 2016.

The easy reason for the split is politics. Graham does not face re-election to his Senate seat until 2020 and McCain must get through a Republican primary in August. While the Arizona Republican is heavily favored to win his primary, his state’s GOP voters gave Trump nearly 50 percent in a blowout for the real estate mogul in the state’s March presidential contest.

Then, McCain faces a general election challenge that could be the “race of my life,” as he described it at a fundraiser that was taped by an attendee and leaked to Politico. Despite the low profile of the likely Democratic candidate, Rep. Ann Kirkpatrick, McCain suggested that Trump’s divisive rhetoric and policies toward immigrants would make his race difficult because of his southwestern state’s heavy Latino bent.

“At the end of the day, John’s going to have to run a campaign where he outperforms Donald and I think he will,” Graham said.

But the Graham-McCain split on Trump has deeper roots than mere political calculation. At some level, it’s personal.

[Peak Lindsey Graham: Trump is a ‘nut job’, ‘a loser as a person’ and ‘ill suited’ to be president]

Graham used his presidential candidacy to warn Republicans about the danger of Trump. Back in July, when most Republican contenders were playing nice with Trump in expectation of his candidacy fizzling, Graham warned that Trump’s strident anti-immigrant policies would “kill my party”.

While he never got on the main debate stage, Graham used one of his under-card appearances to skewer Trump’s views on the Islamic State: Trump “clearly doesn’t understand this war and how to win it”.

The comment made Graham a punching bag for Trump, who cited Graham’s poor campaign as an example of how the Republican establishment was bankrupt. In a February swing through Graham’s South Carolina, Trump called the state’s senior senator “a disgrace” and “a nut job” and “one of the dumbest human beings”.

Seeing Trump up close left its mark on Graham, in a way that McCain never quite saw.

“I was in the contest — he wasn’t. That’s the main difference, I think,” Graham said, explaining McCain’s ability to support Trump.

McCain came under fire from Trump along the way, including a moment last July when the businessman suggested McCain wasn’t a war hero because he got shot down over Vietnam. This week, Trump declined to apologize for the remark.

Ultimately, McCain feels a sense of loyalty to Republican voters. As a former presidential nominee himself, it would be very hard for him to reoncile with the Republicans who reject the outcome of those primaries.

“There’s no illegal votes that I know of,” he said of Trump’s victories.

McCain also couldn’t recall the last time he and Graham split on any issue, thinking for five seconds and then guessing there must have been some divide on a local issue that pit Arizona’s interests against those of South Carolina.

“We’re not Siamese twins, we’re not joined at the hip,” he said. “There are certain issues that we just simply disagree on. That has no effect on our relationship.”