Twitter is proving to be Trump's Achilles' heel

It was just last week that Donald Trump had the finest moment of his short presidency -- his address to a joint session of Congress. Even many of his harshest critics praised his speech or reluctantly conceded that it was "presidential."

The collective response from Republicans was no mere sigh of relief. It was more like the discovery that some vital biblical prophecy had been fulfilled: The Holy Pivot is here at last! Huzzah and hooray. Give the plebes extra rations of grain and wine, for today we celebrate!

We've had President Trump for a while, but this was Presidential Trump.

The obvious lesson drawn by Republican leaders was: This Trump could get serious things done. Just dialing it back to an 8 or 9 from his usual 11 or 12 (on a 10-point scale) reassures congressional allies, making it much easier for them to carry out his agenda. Acting presidential also undermines conservative critics and, more important, makes Democrats and much of the media look hysterical in their overreactions and calls for "resistance."

And since Trump's most loyal fans will celebrate anything he does, there's no need to pander to them with the greatest hits anymore.

In short, there is no downside and all manner of upside for Trump to play the part of a somewhat sober, serious, responsible president -- even one with an ambitious populist-outsider agenda.

Naturally, Trump opted for Plan B.

Step 1: Destroy all hope on Twitter.

Can you imagine the dread, the slow-burning existential panic that overtakes White House Chief of Staff Reince Priebus and Press Secretary Sean Spicer on Friday evenings?

After Ivanka Trump and Jared Kushner -- who are known to have a calming effect on the president -- head off to honor the Jewish Sabbath, Priebus and Spicer have to white-knuckle it until dawn, gripping their phones with tobacco-stained fingers as they constantly refresh Twitter. They must feel like they work for a kind of werewolf who, in the dark, still hours of the weekend, transforms into a presidency-gobbling beast.

I can almost hear the wail that slipped past Priebus' lips on Saturday morning when, just as the chirping birds provided a hopeful soundtrack for the first light of a new day, the president's tweets started pouring forth.

"Terrible! Just found out that Obama had my 'wires tapped' in Trump Tower just before the victory. Nothing found. This is McCarthyism!" And: "How low has President Obama gone to tapp my phones during the very sacred election process. This is Nixon/Watergate. Bad (or sick) guy!"

There were other tweets, about this deadly serious allegation and, of course, Arnold Schwarzenegger. The president who announced before Congress, "The time for trivial fights is behind us," wanted to make sure the American people knew that Schwarzenegger was leaving "The New Celebrity Apprentice" because of his "pathetic" ratings.

But that info came only after he accused his predecessor of committing a felony, in what would be -- if proved -- one of the great political scandals in American history. It now seems clear that when the president said he "just found out" about this wiretapping, he meant that he had just read about it on Breitbart.com.

There is an enormous amount we do not know, though James Comey, the head of the FBI, and James Clapper, the former director of national intelligence, say the claims are untrue. I think more investigations are in order (including of the leaks plaguing the administration).

But my point is far simpler -- and more obvious. Trump is destroying his presidency one tweet at a time. On Friday morning, Trump reportedly chewed out his senior staff for letting allegations of his campaign's collusion with Russia distract from his post-speech agenda. By dawn the next morning, his tweets had made the Russia allegations a much bigger story and led to Spicer asking Congress to investigate whether his boss was onto something.

The pivot stuff was always false prophecy. Being president has a funny way of making people more presidential. And by day, Trump's White House staff can contain his worst instincts. But all bets are off when he's alone at Mar-a-Lago and the moon calls forth the beast.

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Jonah Goldberg is a fellow at the American Enterprise Institute and editor-at-large of National Review Online.