So a Clinton-Trump matchup is not only thinkable at this point. It looks — and I can’t believe I’m writing this — almost probable. See? I’m guilty of what I warn against. I wrote “almost” because it is so hard to view this campaign as it is, rather than as I might think it should be.

Clinton would be seen, at least initially, as the prohibitive favorite. But she had better not bring a knife to a gunfight. The biggest challenge for her campaign, which is nothing if not professional and by-the-book, would be to recognize what Rubio, Cruz, Jeb Bush, Chris Christie and the rest of the Republicans failed to see: To beat Trump you have to go after him just as viciously — and cleverly — as he goes after you.

Refusing to descend to his level is a grave mistake. You have to get down and dirty, get under his skin, call him names. You have to worry less about running a campaign the nation can be proud of and more about running a campaign that wins, even if it wins ugly.