The Trump candidacy is pure base, and Mr. Trump has not built out from that base, which topped out at about 30%. It’s become obvious that this third of angry conservative voters is volatile. Mr. Trump’s famous support base has eroded, dispersing to the other outsider candidates, Ben Carson and Carly Fiorina.

More important, it is now clear that Mr. Trump is personally incapable of doing what is necessary to expand beyond his early burst of support. The tax plan he released this week, admirable as a broad outline, is supposed to show he’s getting serious. That’s the problem. His core base didn’t want that kind of serious.

Even at the level of performance art, what’s happening now is the slow-motion disintegration of “Trump.” His candidacy is detouring into weird and confusing fights, such as the “boycott” of Fox News. News reports on the Trump candidacy increasingly note remarks from admirers who essentially say: I really like that he tells it like it is, but I’m not sure he’s a good fit for the presidency.

The pace of volatility in contemporary politics is unprecedented, as a 74-year-old Vermont socialist is revealing to the preordained candidacy of Hillary Clinton. That the improbable Mr. Trump could rise and then flatline in so little time is startling but not surprising. What Mr. Trump ought to recognize is that his place in the 2015 moment—his political legacy—is secure, unless he lets it evaporate.