In fact, you have to wonder why, exactly, the Republican establishment is really so horrified by Mr. Trump. Yes, he’s a con man, but they all are. So why is this con job different from any other?

The answer, I’d suggest, is that the establishment’s problem with Mr. Trump isn’t the con he brings; it’s the cons he disrupts.

First, there’s the con Republicans usually manage to pull off in national elections — the one where they pose as a serious, grown-up party honestly trying to grapple with America’s problems. The truth is that that party died a long time ago, that these days it’s voodoo economics and neocon fantasies all the way down. But the establishment wants to preserve the facade, which will be hard if the nominee is someone who refuses to play his part.

By the way, I predict that even if Mr. Trump is the nominee, pundits and others who claim to be thoughtful conservatives will stroke their chins and declare, after a great show of careful deliberation, that he’s the better choice given Hillary’s character flaws, or something. And self-proclaimed centrists will still find a way to claim that the sides are equally bad. But both acts will look especially strained.

Equally important, the Trump phenomenon threatens the con the G.O.P. establishment has been playing on its own base. I’m talking about the bait and switch in which white voters are induced to hate big government by dog whistles about Those People, but actual policies are all about rewarding the donor class.

What Donald Trump has done is tell the base that it doesn’t have to accept the whole package. He promises to make America white again — surely everyone knows that’s the real slogan, right? — while simultaneously promising to protect Social Security and Medicare, and hinting at (though not actually proposing) higher taxes on the rich. Outraged establishment Republicans splutter that he’s not a real conservative, but neither, it turns out, are many of their own voters.

Just to be clear, I find the prospect of a Trump administration terrifying, and so should you. But you should also be terrified by the prospect of a President Rubio, sitting in the White House with his circle of warmongers, or a President Cruz, who one suspects would love to bring back the Spanish Inquisition.

As I see it, then, we should actually welcome Mr. Trump’s ascent. Yes, he’s a con man, but he is also effectively acting as a whistle-blower on other people’s cons. That is, believe it or not, a step forward in these weird, troubled times.