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How in the world is John McCain still supporting Donald Trump anyway?

Sen. John McCain, R-Ariz., rings his bell as he volunteers with Salvation Army at a local mall on Monday, Dec. 21, 2015, in Phoenix. (AP Photo/Ross D. Franklin)

Sen. John McCain (R-Ariz.) still counts himself a Donald Trump supporter -- at least, last we checked. Even in roundly criticizing Trump's attack on the parents of a fallen Muslim U.S. soldier, he reiterated Monday that he will support Trump.

Now we have Trump's response to the criticism and the continued support: the Republican nominee told The Post on Tuesday he's not necessarily backing McCain or House Speaker Paul Ryan in their contested primaries.

McCain has options. He can keep supporting a guy who won't support him. Or he can withdraw his Trump endorsement at the risk of it looking like petty politics. (We didn't say these were good options.)

But the most surprising thing about this situation still isn't Trump's lack of support for McCain. It's the fact that McCain supported Trump in the first place.

Sen. John McCain (R-Ariz.) is supporting Donald Trump for president, despite the GOP front-runner's past comments about his service in Vietnam. (Video: Peter Stevenson/The Washington Post)

Very early in his campaign, you might recall, Trump questioned the heroism that completely defines McCain's public and political persona. Here's the exchange with Frank Luntz, per Politifact:

TRUMP: "(McCain) insulted me and he insulted everybody in that room. And I said somebody should run against John McCain, who has been, in my opinion, not so hot. And I supported him for president. I raised $1 million for him. That’s a lot of money. I supported him, he lost, he let us down. But he lost and I never liked him much after that 'cause I don’t like losers. But, but — Frank, Frank, let me get to it.
LUNTZ: "He’s a war hero. He’s a war hero …"
TRUMP: "He’s not a war hero ..."
LUNTZ: "He’s war hero."
TRUMP: "He is a war hero ..."
LUNTZ: "Five and half years in a Vietnamese prison camp …"
TRUMP: "He’s a war hero because he was captured. I like people that weren’t captured. So he’s a war hero …"
LUNTZ: "Do you agree with that?"
TRUMP: "He’s a war hero, because he was captured, okay? I believe, perhaps, he’s a war hero. But right now he said some very bad things about a lot of people. So what I said is John McCain, I disagree with him that these people aren’t crazy."

Again, this is McCain's calling card. It's in the first line of his bio -- along with "U.S. senator" and "2008 Republican presidential nominee." It's also a harrowing experience that he went through personally -- only to have Trump question how heroic it truly was. This was basically the most personal attack possible.

Endorsing Trump was surely a difficult call for many a Republican. But perhaps for nobody was it as difficult as for McCain. Even as Arizona's other somewhat-moderate Republican senator, Jeff Flake, has been pretty blunt in his Trump criticism and withheld his support, McCain has fallen in line.

That could have something to do with the very thing that Trump won't endorse McCain for -- his 2016 primary. If McCain wants to be renominated, it's perhaps best not to alienate the many Arizona Republicans that backed Trump and turn this into a Trump vs. McCain primary.

It's also partly about the general election and McCain's team player status. The party as a whole has made a calculated decision that backing Trump and making sure he's a decent general election candidate who doesn't get blown out is a good thing for all of them. McCain has an unusually tough general election, too, facing Rep. Ann Kirkpatrick (D-Ariz.).

But McCain's endorsement of Trump has come even as his daughter, Meghan, said during last month's Trump-led GOP convention that the Republican Party is dead and as his granddaughter, Caroline, said Trump's questioning of her grandfather's military service was a deal-breaker for her.

And it came after he, in March, echoed Mitt Romney's concerns about Trump's fitness to manage American foreign policy -- the focal point of McCain's decades of policy work in the Senate.

“I share the concerns about Donald Trump that my friend and former Republican nominee, Mitt Romney, described in his speech today," McCain said at the time. "I would also echo the many concerns about Mr. Trump’s uninformed and indeed dangerous statements on national security issues that have been raised by 65 Republican defense and foreign policy leaders."

It comes even as Trump yet again seems to gloss over the sacrifice of a soldier who gave something -- in this case, Army Capt. Humayun Khan's life -- in service to his country.

Here's what politicians from both parties are saying about the row between Donald Trump and a man whose son was killed serving in Iraq. (Video: Monica Akhtar/The Washington Post)

Here was a guy who is perhaps weakest on the issue that the Arizona senator most cares about, a guy who McCain's own family has said is unacceptable, a guy who has now questioned the very substance of the former presidential nominee's political brand, in a way, twice. And yet McCain endorsed him.

Trump has made plenty of Republicans second-guess their decisions to endorse him; Ryan certainly has a claim on that.

But for nobody has this impossible choice been so impossible as it was for John McCain.