It's a role McCain has long played, but it's taken on a more serious tenor in the era of President Trump. Already this year, the senator has logged more than 75,000 miles to more than 15 nations across three continents, according to his staff.
McCain's journeys to those far-flung places also drains the octogenarian — and it is beginning to show when he returns to the Capitol. He won't publicly admit this, but some friends suspect his awkward performance at the Intelligence Committee's hearing with James B. Comey, the fired FBI director, could be traced to McCain's near-constant global travels whenever Congress takes a break.
Something seemed off about McCain on Thursday, and social media took notice (even Fox News blasted a headline, "Partisans agree … McCain was confusing"). McCain tried to ask Comey questions about how he got fired and how he made the decision last year to close the investigation into Hillary Clinton's handling of classified information while she served as secretary of state.
At times, it seemed as if the 2008 Republican presidential nominee was equating the two investigations — one that was closed almost a year ago, and another that seems to still be expanding. The line of questioning seemed to many to be an attempt to defend Trump.
That certainly is not the role McCain is playing around the world.
More aptly, McCain is serving as a shadow secretary of state, trying to clean up or refute statements and positions that Trump has made.
"When Senator McCain is in the room, there's just a different level of respect," Sen. Christopher A. Coons (D-Del.), who traveled with McCain last week, said in an interview. "His mere presence reassures our allies in the Asia Pacific and the Northern Atlantic."
Look at where he traveled and what he said over the 10-day break Congress took around the Memorial Day holiday. From Australia to Vietnam to Singapore, McCain tried to shore up frayed alliances. He landed in Australia several months after Trump had an embarrassing clash with Prime Minister Malcolm Turnbull over a refugee agreement. McCain sat on the floor of the Parliament of Australia during "Question Time," an honor reserved usually for visiting heads of state or foreign ministers.
And Turnbull and two former prime ministers attended a McCain speech toward the end of the visit. The 80-year-old senator didn't hold back his contempt for Trump's behavior toward such a long-standing ally.
"I come to Australia at a time when many are questioning whether America is still committed to these values. And you are not alone. Other American allies have similar doubts these days. And this is understandable," he said. "I realize that some of President Trump's actions and statements have unsettled America's friends. They have unsettled many Americans as well."
He went on to detail the state of American politics, the rise of nationalist populism, legislative dysfunction and hyper-partisanship. He expressed understanding for those who doubt Trump, but he assured them that the president's national security team was experienced and fully invested in the nation's traditional role of global force for democracy.
Unlike some senators who struggle to get the best meetings abroad, McCain always prompts presidents and prime ministers to clamor for an audience. "Because they want to hear what he has to say," said Sen. John Barrasso (R-Wy.), a frequent travel companion. "He is a reassuring figure around the world."
Barrasso and Coons joined McCain in Vietnam, where McCain is now a revered political figure after having been shot down as a Navy fighter pilot and held captive 5 1/2 years. In the Senate in the 1990s, McCain and John Kerry (D-Mass.), another Vietnam War hero, led the effort to normalize relations with Vietnam.
After the more recent delegation's first day there last week, the leading Vietnamese newspaper ran a front-page story with a headline "Senator McCain and friends meet with national leaders," Coons recalled. No mention of the other lawmakers on the trip.
"As a face of Congress around the world, he would be the prime minister of Congress," Barrasso said.
Last week was only the latest whirlwind itinerary for McCain.
Two days after Christmas, McCain began a swing through several Baltic nations that view Russia's incursions into Ukraine as an existential threat to their freedom.
Ten days after Trump won the election, McCain led a bipartisan delegation to the Halifax International Security Forum, where NATO allies were panicked by Trump's declaration that the security alliance might be "obsolete" amid his softening toward Russia.
"We were reassuring people, in some cases scraping them off the ceiling," Coons said of those meetings.
McCain sent the clearest signal of the role he was assuming as international fixer at a February speech in Munich, but he has continued this approach in many other lower profile appearances around the globe.
In one week in April, McCain hit seven tiny nations across southeastern Europe that collectively form a key region — and serve as a transit point for Islamist militants heading to and from Syria. In a speech in Kosovo, he again never mentioned Trump, but he assured leaders that they still had friends in Washington.
"You may have heard that we occasionally have some disagreements back in Washington. But I want you to know that members of the United States Congress in both political parties are fully committed to working with you," McCain said.
The schedule is grueling, even for younger colleagues. Coons, 53, marveled at how McCain squeezed in a late-night meeting with Defense Secretary Jim Mattis at a security conference in Singapore just as he cut short his trip to fly back to tend to his wife, Cindy, who had broken her arm in a fall.
That type of travel will wear down the healthiest of senators, even one whose mother, Roberta McCain, is alive and well at 105. After his embarrassing moment questioning Comey, McCain issued a formal statement blaming his performance on staying up late to watch an Arizona Diamondbacks game on the West Coast.
There's no indication he will slow down, not with so many allies who don't understand what Trump means with his tweets and brusque diplomacy.
"We need your patience and your understanding. We need your commitment to our common interests and ideals. We need you to have faith in America and in the enduring value of our alliance," McCain told the Australians. It's a message he's likely to repeat in the coming weeks and months.