Over the weekend, Donald Trump held a rally in Phoenix, Arizona, that attracted several thousand people. He shared the stage with the father of a man who was killed by an undocumented immigrant—and Trump continued his rant against illegal immigration that began when he launched his campaign and started to surge in the polls. Not every Republican in Arizona was pleased with Trump’s visit. Senator John McCain, the Party’s Presidential nominee in 2008, reacted to the event with dismay.

“It’s very bad,” McCain, who was eager to talk about Trump, told me on Monday when I stopped by his Senate office. The Senator is up for reëlection in 2016, and he pays close attention to how the issue of immigration is playing in his state. He was particularly rankled by Trump’s rally. “This performance with our friend out in Phoenix is very hurtful to me,” McCain said. “Because what he did was he fired up the crazies.”

McCain, who has long supported comprehensive immigration reform and was a member of the so-called Gang of Eight that successfully pushed immigration legislation through the Senate in 2013, has been at war with the far right in Arizona for years. “We have a very extreme element within our Republican Party,” McCain said. He then noted that he was personally censured by Arizona Republicans in January of 2014 and has been fighting to push out the extremists in the state G.O.P. ever since. “We did to some degree regain control of the Party.”

But McCain fears that Trump may be reversing those gains. “Now he galvanized them,” McCain said. “He’s really got them activated.”

McCain probably has more experience navigating the issue of immigration than any other national Republican politician. He has fought off right-wing challengers in Arizona primaries and run twice in G.O.P. Presidential primaries. He has occasionally reined in his enthusiasm for an immigration-reform plan that would include a pathway to citizenship for millions of undocumented immigrants—he hedged a bit during the 2008 campaign—but he has never abandoned the policy.

Many Republicans assume that Trump’s current position at the top of national polls won’t last, and McCain, who said that he last met Trump many years ago, pointed out that conservatives are starting to learn more about Trump’s liberal past. “He was a big Democratic supporter,” he said. “Some of this stuff is going to come out: he gave more money to Democrats than Republicans; he had Hillary Clinton at his wedding. You know, he’s attacking Hillary Clinton after she was in the front row of his—I don’t know which wedding it was.” (Trump has been married three* times.)

But McCain worried that Trump might have more staying power than many political analysts assume. And, even if he slips in the polls, Trump’s attacks on immigrants and his focus on the porous border will have a warping effect for Republicans.

“We’ll see how this plays out, but there is some anger in my state,” McCain said. He mentioned the continuing challenges of border security that were vividly highlighted when tens of thousands of Central American minors crossed into America last summer. “People who otherwise might be more centrist are angry about this border situation.”

McCain is an ardent backer of his good friend Senator Lindsey Graham, who is languishing in the G.O.P. Presidential primary polls. He noted that Graham has been one of the few Republicans to condemn Trump in strong terms. On Sunday, Graham said on CNN, “I think [Trump]’s a wrecking ball for the future of the Republican Party with the Hispanic community, and we need to push back.” He added that Republicans “need to reject this demagoguery. If we don’t, we will lose, and we will deserve to lose.”

McCain, who is eighteen years older than Graham, sounded like a proud father. “Lindsey said this is a moral test for our party. He put on a very strong performance,” McCain said. “Of course, Lindsey was one of the eight of us who negotiated immigration reform. Lindsey never backed away from it.”

McCain, who had a testy relationship with Senator Marco Rubio, another member of the Gang of Eight who is running for President, couldn’t resist adding, “Rubio backed away from it.”

I noted that Rubio, like many other Republican politicians, has been hard to follow on the issue and no longer supports the compromise approach that the Gang of Eight took in 2013: combining a pathway to citizenship and tough new border measures in a single bill. McCain licked his finger, held it up in the air, and laughed.

“You know that old song from before you were born?” McCain said, speaking of the Bob Dylan classic “Subterranean Homesick Blues.” “You don’t need a weatherman to know which way the wind blows.”

* An earlier version of this piece said that Trump had been married four times.