Another similarity between Mr. Trump and Mr. Berlusconi is that they both bring to politics a flair for seduction that served them well in their previous careers in construction, television and entertainment (and elsewhere, or so it’s said). They know their message ought to be reassuring and easily digestible. Both are convinced that, in an era obsessed with appearances, image is key.

So, especially for those on the fence about supporting Mr. Trump (according to polls, a surprisingly large number of Americans): Don’t buy. Behave as you would with a chatty car salesman. Ask lots of questions, and expect proper answers. Open the trunk, check the brakes. Just don’t ask for a test drive — Mr. Trump may lock the door, speed away and drive you into the next wall.

Finally, don’t dig yourself into a hole. Margaret Thatcher’s classic acronym, TINA — There Is No Alternative — says it all about the attitude of many voters. Before choosing what they think is right, they consider what they believe to be possible. Mr. Berlusconi lasted as long as he did for lack of alternatives; Mr. Trump is banking on the creation of a self-fulfilling narrative, that even if he is not the best candidate, he is the only viable candidate.

But of course, that’s true only if the public and the media let it be true — the Republicans have 16 hats in the ring, and several of them, from across the right-wing spectrum, offer a saner option for the presidency than Mr. Trump. All it takes is the establishment’s rallying behind one of them, but they have to do it soon.