By now, it's a given: Nearly every time he addresses supporters or gets a chance to list his accomplishments, President Trump takes a dig at Sen. John McCain (R-Ariz.).

Specifically, Trump can't stop mentioning — and mocking — McCain's surprise “no” vote last summer that ended Republicans' efforts to gut Obamacare.

It's clear that Trump blames McCain for his party's failure to repeal Obamacare — or at least sees him as a reliable scapegoat. Trump has brought it up three times in the past four days alone, as he ramps up his campaigning ahead of November's midterm elections.

In the process, the president has displayed a remarkable lack of sensitivity to McCain's very serious health issues. McCain was diagnosed with brain cancer a year ago and hasn't been in the Senate since December to cast a vote.

Yet Trump still frequently frames McCain as a spoiler to one of the central pieces of the president's agenda.

“We were very surprised by one gentleman,” Trump said Tuesday while speaking to reporters at the White House.

And at a rally in South Carolina on Monday: "[H]e campaigned on repealing and replace, we had all the votes, and perhaps he was grandstanding, who knows what he was doing? But you know what? He said, 'No, no.' Everybody said, 'What the hell happened?' He's been campaigning for eight years — repeal and replace. And he didn't do that.”

And at a rally in Nevada on Saturday: “Nobody talked to him. Nobody needed to, and then he walked in: Thumbs down.”

And at a rally in Minnesota on Wednesday: “We had a gentleman way into the morning hours go thumbs down. He went thumbs down.”

It goes all the way back to a rally Trump held in McCain's home state of Arizona in August shortly after the vote and just a month after McCain announced he had brain cancer.

If anything, since the diagnosis, Trump and his allies have ramped up their criticism of McCain. The most outrageous instance of that was in May, when a White House aide went so far as to make light of the senator's health behind closed doors. The White House refused to publicly apologize when those comments leaked.

McCain has mostly kept his recent criticism of the president to policy. But his daughter Meghan McCain, who has become a kind of spokeswoman for the family, recently called Trump's repeated, onstage mockeries of her father “gross.”

Trump does have reason be wary of McCain. If there is a battle for the soul of the Republican Party, McCain is still the face of its anti-Trump wing, even from home in Arizona.

McCain has criticized Trump's foreign policy as “half-baked spurious nationalism,” urged opponents of Trumpism to “fight” it and tried to torpedo Trump's pick to lead the CIA by urging his colleagues to vote against her.

But Trump's mockery of McCain rings hollow in other ways. McCain has been home in Arizona all year to treat his cancer, and it's not clear when he will return to Washington. The form of brain cancer he is battling is a particularly aggressive one.

There are also lots of reasons an Obamacare repeal failed; McCain's opposition was just one of them. He wasn't the only Senate Republican to vote against it, but his “no” was the most surprising and dramatic. His thumbs down on the floor of the Senate toward the end of a late-night vote elicited gasps among those present. 

At the time, McCain said he didn't like the backdoor process by which the bill came to form. In his new book, “The Restless Wave,” McCain wrote that he thought the bill didn't do much beyond undoing current law: “I’m not sure we’ll ever agree on a replacement, and so perhaps all we can do is try to fix parts of Obamacare.”

Trump immediately started blaming Republican leaders in Congress. He has since settled on solely blaming McCain.  

It seems the one sliver of respect that Trump has afforded McCain is to avoid directly naming the senator when mocking him.

Trump has been grudging about even that slight show of restraint. At a rally in Arizona a month after the Obamacare vote, Trump said this of McCain: “I will not mention any names. Very presidential, isn’t it? Very presidential.”

Trump's slights come in the form of silence, too. When he gave the commencement address at the U.S. Naval Academy in May, he didn't mention McCain, who is one of the academy's most notable alumni.

Inadvertently or not, McCain has played an outsize role in Trump's political rise. Barely a month into his presidential bid, Trump touched what many in politics thought was a third rail when he described McCain as “not a war hero.”

Trump has somehow turned mocking one of the Republican Party's living legends into a political tagline that usually appeases any Trump-friendly crowd he's talking to. Using the party's ailing figurehead as a foil is just one more political norm Trump has smashed as president.