On his first full day as secretary of state, former Exxon Mobil C.E.O. Rex Tillerson called on his new employees at Foggy Bottom to band together in the face of sweeping change, much of it ushered in by his own new boss, Donald Trump. “I know this was a hotly contested election, and we do not all feel the same way about the outcome,” Tillerson said, addressing the roughly 236 pound elephant lurking across town in the White House. “We cannot let our personal convictions overwhelm our ability to work as one team.”

Arizona Senator John McCain, meanwhile, appears unencumbered, for the first time in years, by any desire to let politics get in the way of his own personal convictions. The 80-year-old, 2008 presidential runner-up has taken to his newfound role as Republican Trump antagonist with something approaching glee, offering endless droll recriminations of the administration. But he is also approaching the job with deadly seriousness, calling up foreign leaders and flying around the world as a sort of shadow secretary of state, contravening White House pronouncements and reassuring jittery allies.

Shortly after reports leaked that Donald Trump had insulted Australian Prime Minister Malcolm Turnbull in their first conversation, saying it was his “worst call by far” and that he would not honor an agreement made by the Obama administration to resettle 1,250 refugees, McCain rushed to assure Turnbull that the U.S. and Australia remained committed to each other.

“Australia is one of America’s oldest friends and staunchest allies,” he said in a statement, praising their century-old military alliance and trade relationship. “In that spirit, I called Australia’s ambassador to the United States this morning to express my unwavering support for the U.S.-Australia alliance.”

It’s not the first time that McCain has played the part of senior diplomat-in-exile, circumventing the White House to clean up Trump’s mess. At the end of December, the Republican traveled to Estonia to reassure America’s Baltic allies that the United States remained committed to the NATO alliance, and later flew to Ukraine with Senators Lindsey Graham and Amy Klobuchar, where he spent New Year’s Eve huddled with Ukrainian marines in the village of Shyrokine. “We stand w/ them in their fight against #Putin's aggression,” he tweeted, tacitly affirming his opposition to Trump’s solicitous Russian agenda. He has repeatedly criticized Trump’s openness to thawing relations between the two countries, has condemned the possibility of lifting economic sanctions, and has called for congressional hearings to investigate Russia’s interference in the 2016 election.

McCain, the chairman of the Senate Committee on Armed Services, has also spoken out about what he sees as early signs of military mismanagement by the Trump White House. On Sunday, McCain said he is worried about the way Trump had restructured the National Security Council, and on Thursday, he rebuked Trump’s handling of a recent raid in Yemen, where a U.S. soldier died during the administration’s first military operation. McCain has called for a full hearing on the mission, during which, one official told NBC News, “almost everything went wrong.”

If Trump’s recent interactions with foreign leaders are any sign, McCain will be plenty busy for the next four years—and Trump will need all the help he can get. In addition to alienating Turnbull, Trump recently raised tensions with Mexico by demanding that Mexican President Enrique Peña Nieto pay for Trump’s infamous border wall, prompting Nieto to cancel an upcoming trip to the United States. He has also degraded relations with Germany, another key ally, by effectively accusing Chancellor Angela Merkel of manipulating currency. Trump‘s recently-imposed ban on refugees and certain travelers from seven majority-Muslim countries, which McCain fiercely criticized, has also provoked worldwide outrage.