President Trump's Twitter attacks on the late Sen. John McCain, R-Ariz., over the weekend — including his retweet of one supporter who explained that she "hated" the late senator — invite the question: Why does Trump harbor such a special hatred for the long-deceased McCain?

Well, Trump would suggest it's three factors. First, there's McCain's transmission of the so-called Steele dossier to the FBI. Compiled by Christopher Steele, the former British intelligence officer, that dossier suggested Trump's involvement in various nefarious and lewd activities. Then there's McCain's decision to vote down the health care bill sponsored by his friend Sen. Lindsey Graham, R-S.C. That bill, which would have passed had McCain supported it, would have replaced the Affordable Care Act, also known as Obamacare. Finally, there's McCain's sustained criticisms of Trump during the first 18 months of his presidency.

I think these issues play into Trump's deep dislike for McCain, but there's an even more basic rationale for his fiery fury: McCain encapsulates everything Trump despises most in American politics.

Most obviously, McCain was and is an eternal marker for Republican disenchantment with the Trump presidency. Trump is gravely aggravated by this disenchantment because he believes he has earned a right to be treated as the modern Caesar of contemporary Republicanism, a leader entitled to absolute, unflinching loyalty. Trump simply cannot rationalize why McCain was able to reject his presidency and somehow also retain the respect of Republicans.

[Related: Lindsey Graham says John McCain's legacy won't be 'diminished' after Trump attacks the late senator]

But it's not just that McCain refused to kiss the emperor's ring. It's that McCain knelt to a different ideal outside Trump's court, and one for which Democrats and the media at times praised him. Trump has little regard for military service in and of itself. He judges service by obvious outcomes alone and not sacrifice for a cause.

This takes us to the context of March 2019. Increasingly enraged by the media's refusal to bow to him, Trump feels the continued celebration of McCain as just more salt in the ego wound.

Yes, McCain obviously deserves better than this scorn. But Trump's rage against him is unlikely ever to waver.