What did Donald Trump do today?

He went out of his way to insult a terminally ill man.

Today, Trump signed a routine defense budget bill, formally titled by Congress the "John S. McCain National Defense Authorization Act for Fiscal Year 2019," as a gesture of respect to Sen. John McCain who is gravely ill with cancer. McCain was a Navy pilot who spent more than five years as a prisoner of war during the Vietnam War.

Trump refused to refer to the bill by its name, and did not mention McCain once during 29 minutes of remarks. Neither did Vice-President Mike Pence, or anyone else who spoke at the signing ceremony.

Trump loathes McCain, and has since before he became president. On the campaign trail, he famously mocked McCain's years of torture and captivity by saying "He's a war hero because he got captured? I like people who weren't captured." (The audience at the conservative Family Leadership Summit gasped and booed.) Trump later added, "He lost and he let us down. I don't like losers." 

Their relationship deteriorated further during Trump's term. McCain voted against Trump's attempt to repeal the Affordable Care Act, more on principled grounds than any fondness for "Obamacare," and has called Trump's flirtation with Vladimir Putin "disgraceful." Trump, for his part, has bitterly blamed McCain for the failure of his legislative agenda, and pointedly refused to discipline a White House staffer who made a joke about McCain dying.

Why should I care about this?

• Presidents who can't set aside personal grudges in order to do their jobs can't do their jobs.