Sen. John McCain receives the Liberty Medal from the National Constitution Center in Philadelphia in October. (Matt Rourke/AP)

President Trump has given us plenty of opportunities to learn how sensitive he is to criticism. And Trump seems particularly sensitive to his chief Republican antagonist in the Senate, Sen. John McCain (Ariz.).

The president still hasn't forgotten that night almost a year ago when McCain gave a thumbs-down vote, ending Republicans' hopes of repealing Obamacare. While in Dallas last week to rev up his base at the National Rifle Association, Trump couldn't resist a dig at McCain, who is at home in Arizona being treated for a brutal form of brain cancer.

All that is why it's a safe assumption that Trump views what McCain's doing this week as a giant middle finger from back home in Arizona. Intentions are hard to decipher, but McCain is poking Trump right where it could hurt the president the most this week, and it's elevating the two politicians' battle for the soul of the Republican Party.

McCain is publicly engineering a bid to tank Trump's pick to lead the CIA, Gina Haspel. He said in a statement Wednesday that the fact that Haspel refused to say in her confirmation hearing that torture is wrong “is disqualifying.”

McCain is not expected to travel to Washington to vote against Haspel, but his words carry weight as a kind of Senate dean on war and torture. McCain was tortured as a prisoner of war during the Vietnam War and was a vocal opponent after the attacks of Sept. 11, 2001, of waterboarding, which is now banned as an interrogation practice.

On Thursday, Sen. Jeff Flake (R-Ariz.) told reporters that he's undecided on Haspel, thanks in part to McCain. That could be enough to sink her confirmation or at least force a tie-breaking vote by Vice President Pence.

Then there's McCain's book, which seems to be poking at the touchiest topics for Trump on the Russia investigation. McCain's memoir is out later this month and is getting leaked in dribbles. Much of what we've heard so far definitely helps fuel a narrative in Trumpland that McCain is out to get him.

The Daily Beast published an excerpt Wednesday of McCain writing that he got hold of a dossier written by an ex-British spy alleging Trump-Russia collusion. McCain passed it to the FBI director at the time, James B. Comey.

“The allegations were disturbing, but I had no idea which if any were true,” McCain writes, according to the Daily Beast. “I could not independently verify any of it, and so I did what any American who cares about our nation’s security should have done.”

The FBI used the dossier as part of its application to get a warrant to spy on one of Donald Trump's advisers during the presidential campaign.

President Trump has seized on the dossier — despite legal experts disagreeing with his assertion — to claim that the FBI and Justice Department are out to get him. It's not hard for him to now draw that line back to McCain.

Elsewhere in the book, McCain compares Trump to a despot.

And those are just excerpts.

We don't need to read a book to know how McCain feels about Trump. Since being diagnosed with brain cancer last summer, McCain has indirectly blamed Trump for the deaths of innocent women and children in a chemical attack in Syria. He's urged his fellow senators to remember that they are not Trump's “subordinates,” and he told cadets at the U.S. Naval Academy that they “have to fight” against Trumpism.

Trump's supporters certainly think McCain's flipping them off from back home. They are punching back without regard for the senator's potentially fatal illness, even trying to leverage it as an attack line.

“I think McCain comes off as a very bitter individual,” Corey Jones, a state director for a pro-Trump political organization, the New Right, told my Fix colleague Eugene Scott this week, even before McCain opposed Haspel's confirmation. “Trump made some strong remarks toward Sen. McCain during the election, and McCain can’t seem to move on. McCain should worry less about who he is and isn’t inviting to his funeral, and worry more about repairing his damaged legacy.”

Fox Business guest Thomas McInerney went there Thursday afternoon on McCain's prisoner of war experience, alleging that it worked on McCain. (McCain is widely regarded as a war hero for his time in Vietnam.)

(Update: McInerney is a former paid analyst for Fox, not a current one, and host Charles Payne issued an apology for McInerney's statement on the show.)

This whole back-and-forth is broader than two politicians and their supporters, of course. McCain and Trump represent two rapidly diverging wings of the party. The old guard and the new guard, the globalists and the nationalists, the carefully thought-out foreign policy preached by McCain and the gut-check foreign policy practiced by the sitting president.

McCain would argue he's saying what's necessary for the good of the country. But add it all up, and it's easy to see how a criticism-averse president could and probably is coming to the conclusion that McCain, as he battles cancer and becomes more reflective, is out to get Trump.