Republican Sen. John McCain may be thousands of miles from Washington, receiving cancer treatment at home in Arizona. But his influence in the nation’s capital has hardly diminished.

This week, Senator McCain, along with fellow Sen. Chris Coons (D) of Delaware, unveiled a new bipartisan attempt at an immigration compromise that would protect so-called “Dreamers” from being deported while beefing up border security.

Last week, as Republicans on the House Intelligence Committee prepared to release a memo detailing alleged abuses in the surveillance of a Trump campaign official, McCain weighed in with a sharply critical statement, saying: “The latest attacks on the FBI and the Department of Justice serve no American interests – no party’s, no president’s, only Putin’s.”

And when it comes to the Senate Armed Services Committee, Chairman McCain is still very much in control. As Sen. James Inhofe (R) of Oklahoma, who is running committee meetings in McCain’s absence, told C-SPAN: “He’s calling the shots.”

Even without all this activity from afar, however, McCain’s presence would be strongly felt – because of his profound influence on many of his colleagues. In recent years, the Arizona senator has purposefully mentored a new squadron of defense hawks on the Hill, most notably from the large Republican class of senators that took office in 2015, when he assumed the committee chairmanship. Seven of them landed on Armed Services.

“All these senators come in, and in a sense, they’re their own independent island. But John McCain has spent a lot of time – and this is the sign of a great leader – trying to mentor people,” says Sen. Dan Sullivan (R) of Alaska, elected in 2014. “He went out of his way to work with a lot of members of my class.”

During a time when the Republican party has been pulled in a notably more isolationist direction, McCain has remained a staunch advocate for US influence in the world – and his committee has embraced that worldview. The annual National Defense Authorization Act, which sets the military budget, passed unanimously out of McCain’s committee last year with a significant increase in military spending (Congress is now tussling over appropriating those funds).

As one Capitol Hill staffer puts it, McCain has helped a new generation of senators appreciate “the importance of American leadership – and American military strength as part of that leadership.”

J. Scott Applewhite/AP Known as a "defense hawk," Sen. John McCain (R) of Arizona chairs the Senate Armed Services Committee – and has played an active role in mentoring many of its newest members.

“Mentor” is not a word one might readily associate with the fiercely independent, blunt, and at times hot-tempered McCain. But it comes up again and again in interviews with committee members.

“Literally, my first month in the Senate, he reached out,” says Senator Sullivan. McCain approached him and asked whether the lieutenant colonel in the Marine Reserves would like to focus on security in Asia. “He came up to me [and said], ‘You know Dan, we don’t really have the next generation of senators focusing on foreign policy and national security in Asia-Pacific. If you’re interested, I want to help you with that.’ ” The freshman senator jumped at the offer, especially given Alaska’s proximity to the region.

Demanding accountability

Like McCain, many committee members had military experience before they got to the Senate. But they say they’ve learned things from him – such as the importance of demanding accountability from officials in public and private settings.

Last October, for instance, at a hearing on Afghanistan, McCain minced no words when Secretary of Defense James Mattis and Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford appeared before the panel. In August, President Trump had announced that he would send a few thousand more US troops to Afghanistan, but held back on details.

“In the six weeks since the president made his announcement, this committee – and the Congress, more broadly – still does not know many of the crucial details of this strategy,” McCain chided. “This is totally unacceptable. I repeat: This is totally unacceptable.”

And he readily put the squeeze on the administration by holding up Pentagon nominees to get more information on strategy and other issues, such as four American troops killed in Niger last year. He has since lifted those holds.

“John is able to ... focus real passion, and communicate that passion to people so that they get it done,” says Sen. Jack Reed of Rhode Island, the Democratic ranking member on the committee. “It’s not like, ‘Well, there’s another request, we’ll put that on the list.’ It’s like, ‘We better get this done. Senator McCain is passionate about this.’ ”

Senator Reed notes that McCain runs his committee in a “profoundly bipartisan” way, letting everyone have their say – which has often helped members reach a consensus.

On overseas trips that often have a reputation for being a bit of a forced march, McCain’s heavyweight status has helped open doors for many of his colleagues.

“When you travel with Senator McCain, you see the best and you get to meet the best. It’s a real learning experience,” says McCain’s best friend in Congress, Sen. Lindsey Graham (R) of South Carolina.

Every minute on these trips is put to use. While lawmakers on other congressional delegation tours – or “codels” – may sit back during flights overseas with earbuds on, McCain sees it as a time to debate the issues: What to do about the Israelis and Palestinians, or Syria?

“He goes from dawn to dusk,” says Sen. David Perdue (R) of Georgia, a former Fortune 500 chief executive officer who traveled with McCain to Afghanistan and Pakistan to visit troops over a July 4 break.

Visiting the ‘Hanoi Hilton’

Sen. Joni Ernst (R) of Iowa recalls a trip with McCain to Vietnam that included Sullivan and Reed. They visited the “Hanoi Hilton,” the prison-turned-museum where McCain was held captive and tortured.

Sitting on the sofa in her Senate office, the former lieutenant colonel and Iraq War veteran had to briefly gather her composure when talking about the trip – and the man.

“It’s hard,” she says, “to see him go back there.” To ease the emotion, she recalls, McCain employed some “dark” humor. Passing photos of former prisoners playing volleyball and standing in front of a Christmas tree, McCain quipped: “It was just like a vacation.”

Later that evening, at one of his favorite restaurants, the former Navy fighter pilot shared stories from his captivity that Senator Ernst had never heard before. “It ... just demonstrates his ability to overcome adversity that most of us cannot even begin to imagine,” she says. He was “able to come out from that and forgive the people that did so many bad things to him.”

Sharing McCain's worldview does not mean sharing all of his political views. Sen. Tom Cotton (R) of Arkansas, a combat veteran who served in Iraq and Afghanistan, might side with the chairman on issues like Iran and military spending, but he’s firmly in Mr. Trump’s camp on immigration. Sullivan has differed with McCain on healthcare.

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Ernst remembers a disagreement she had with McCain over funding. “He had words with me,” she laughs. Ernst persisted in trying to clear the air, and eventually he came around saying, “‘Joni, I can’t stay mad at you.’” While they don’t always agree, Ernst says they “typically do.”

Senators like Cotton, Sullivan, and Ernst “will be in the McCain camp for years to come as far as national security is concerned,” says Senator Graham, strolling through the Senate’s underground passageway on the way to his office. “John has opened doors for them, and mentored them, like he’s mentored me – and it will pay dividends.”