Then came Mr. Mulvaney’s damage-control sit-down on Fox News on Sunday morning. He started out insisting that people were misstating what he had said, but Mr. Wallace was having none of it. “You were asked specifically by Jonathan Karl [of ABC News], Was investigating Democrats one of the conditions for holding up the aid — was that part of the quid pro quo? And you said, ‘It happens all the time.’” This sent Mr. Mulvaney spinning even more furiously, to which Mr. Wallace responded, “I hate to go through this, but you said what you said.” Mr. Mulvaney could protest all he wanted. Mr. Wallace had video receipts.

The acting chief wasn’t much better at trying to explain why Mr. Trump thought it a good idea to hold the Group of 7 gathering at one of his company’s resorts. “At the end of the day, he still considers himself to be in the hospitality business,” Mr. Mulvaney said.

Reviews of Mr. Mulvaney’s Sunday gig were not kind, even among the president’s friends and advisers — one of whom characterized it to Politico as “a self-immolation.” For weeks, word around Washington has been that Mr. Mulvaney has lost favor at the White House. The past few days have kicked speculation into high gear about how much longer he can hang on.

It’s easy to mock Mr. Mulvaney’s missteps. He has emerged as one of Mr. Trump’s top go-to guys for carrying out questionable orders, such as taking the practical steps necessary to withhold Ukraine’s aid money or shove aside career diplomats in favor of political lackeys.

But, in Mr. Mulvaney’s defense, he should never have been put in this position. The guy who’s doing the dirty work should not also be the guy expected to go out and defend it to a roomful of journalists. That’s what a press secretary is for — preferably a professional steeped in the dark art of ensorcelling the Fourth Estate.

Seriously, does anyone think Sarah Huckabee Sanders, in her turn as Mr. Trump’s chief spinner, would have been goaded into admitting a presidential quid pro quo and then admonish everyone for being naïve about that sort of thing? Of course not. She was too adept at dodging, deflecting and flat-out lying to blurt out such an inconvenient truth. If nothing else, she would have pleaded ignorance of the details — which would have been a tougher sell for Mr. Mulvaney given his role as a recurring character in the Ukraine shenanigans.

Here we see a concrete downside to the White House’s slapdash approach to staffing, its disdain for professionalism and this president’s conviction that, because he’s such a communications whiz, he doesn’t need a message team backing him up.