The Arizona senator has publicly criticized the President over his handling of US relations with Australia, his Cabinet picks and his highly contentious travel and immigration ban -- and that's just this week.
Following his re-election last fall, McCain is back to his "maverick" style.
"I'm the one that said Ronald Reagan shouldn't send Marines to Afghanistan, I'm the one that said Donald Rumsfeld should be fired, I've said the general in Afghanistan should be fired," McCain said Thursday, when asked by CNN about his public opposition to Trump. "Look, I have done what I have done right for this country under Republican and Democrat presidents. I will continue to do so. This is not a departure from the way I've conducted myself in the United States Senate."
When CNN asked if he felt like he had more standing than other lawmakers to criticize the president, he deflected with a self-effacing joke.
"Well, my senility has given me the opportunity to be the chairman of the Armed Services Committee," he said, referencing his senior status among Senate Republicans.
McCain blasted the President Thursday for his terse phone call with Australian Prime Minister Malcolm Turnbull
, an American ally. McCain led Trump on a history lesson of the nearly century-long alliance with the country.
"They fought alongside us in wars, including losing over 500 brave Australians in the Vietnam War, which some of us remember," McCain told CNN's Phil Mattingly, offering an implicit dig at Trump, who won five deferments and never served in a branch of the military.
"This, in my view, was an unnecessary and frankly harmful open dispute over an issue, which is not nearly as important as the United States and Australians working together," McCain said.
McCain later questioned Trump's handling of a recent raid in Yemen -- his first military operation, which resulted in the first death of a US soldier on his watch
-- saying he wants a full briefing on the issue. The raid was greenlit by Trump shortly after taking the oath of office, but multiple officials told CNN the mission had been planned months
in advance and had been briefed to then-President Barack Obama.
That McCain is not a fan of the new president is no surprise. The relationship has been strained at best ever since the early days of the presidential election cycle when Trump accused the former POW of not being a war hero
because he was captured.
But McCain's demeanor around the Capitol has changed since he won his re-election bid to the Senate in November. Now the 80-year-old senator needles Trump with a smile, and an almost jovial attitude around the Capitol.
McCain's numerous and repeated questions have had a serious impact for young Trump administration.
His concerns about Secretary of State Rex Tillerson's ties to Russian President Vladimir Putin helped throw the critical nomination into doubt when coupled with Sen. Lindsey Graham and Sen. Marco Rubio's troubles with the former oil CEO. (However, all three ended up voting to confirm him.)
Similarly, McCain blasted Trump's pick for budget director, Mick Mulvaney, last week
because of the South Carolina Republican's previous votes to cut defense spending.
And McCain, along with Graham, dubbed Trump's travel ban a "self-inflicted wound in the fight against terrorism"
in one of the most stark indictments from within Trump's own party.
Other Republicans expressed general concerns with the ban and said they wanted more information, but those concerns largely evaporated this week after Department of Homeland Security Secretary John Kelly briefed lawmakers privately.