As you may have noticed, Republicans recently passed a tax bill. As you may also have noticed, Democrats were aghast. Passing a bill like that on straight party lines! Using a parliamentary maneuver to push through something that could never have survived a filibuster! How could Republicans be so brazen, so immoral, so fiscally irresponsible?

Those of us who remembered saying many of the same things during the passage of Obamacare had to beg them to stop. I mean, we could have been seriously hurt, laughing that hard.

But when I pointed this out, the good citizens of Twitter informed me over and over that this was mere “whataboutism.”

Whataboutism is defending some indefensible action by pointing to some equally indefensible action that was supported, or at least not condemned, by your opponents. (Whataboutism is usually defined as a version of the tu quoque fallacy, attacking the questioner rather than answering the question. It’s also a red herring.) After lobbing a few “What about you?” grenades, you use the resulting chaos to duck uncomfortable questions.

It’s a favorite tactic of our president, whose campaign platform was “What about her emails?” Every time someone brings up the FBI investigation that is creeping closer to the highest echelons of his staff, he is fond of asking, apropos of nothing, why Hillary Clinton’s not in jail.

Whataboutism is currently being deployed by defenders of Roy Moore, who correctly point out that Democrats were obsequiously tolerant of Bill Clinton’s sexual misadventures, without explaining how that in any way ameliorates the charges that Moore preyed on teenagers and groped a 14-year-old.

My interlocutors are right about one thing: “What about” is a lousy defense of anything. If what Democrats did to get Obamacare passed was wrong — or at least, severely inadvisable — then it makes no sense to say “Turnabout is fair play.” Major legislation shouldn’t be passed like this, not least because the parliamentary maneuvers tend to produce crippled bills with major unintended consequences. And so too with Bill Clinton’s sexual predation, or the Democratic defenses of same; they were awful, and no decent person should tolerate that behavior from an ally or an opponent.

“What about …” is not merely a poor rebuttal; it is not even a rebuttal. Hitler liked dogs and Ted Bundy volunteered at a suicide prevention hotline; these are not arguments against dogs or suicide prevention.

Hypocrisy shouldn’t be encouraged, of course. But neither should we try to drive the hypocrites from the public square: There’s no guarantee that we’ll end up with more virtue; we might just end up with a lot more naked, unchecked vice. America will not become a better place if we all implicitly agree that declaring ourselves against sexual abuse, or for an orderly procedure for passing laws, is simply an empty public ritual which no longer has any meaningful content.

However. We should be as judicious in our cries of “whataboutism” as we are about shouting “What about . . . ?”

We should not allow Republicans to deflect attention from their problems by pointing to historical abuses on the other side of the aisle. But neither should we allow Democrats to avoid a necessary reckoning with their own past by pointing the finger at Republicans and shouting “Whataboutism!” What about Roy Moore’s pursuit of teenage girls? What about the Trump campaign’s connections to Russia? What about the procedural insufficiencies and general fiscal recklessness of the tax bill? Yes, what about them? Aren’t they awful?

But also, what about the way Democrats deflected questions about Bill Clinton’s behavior till it was clear that neither Clinton would ever be running for office again? What about Barack Obama’s unwise uses of executive fiat? What about the hubris and the procedural games and the general indifference to our looming entitlement crisis that characterized the passage of Obamacare? When are you going to admit that you were wrong about these things and, more importantly, when are you going to promise not to support this sort of thing in the future?

If you actually care about these things because they are bad in themselves, then you should want them to stop on your own side as much as the other. You should want to understand why you did not do enough to stop your own side’s abuses in the past; you should want to do better next time. You should be prepared to meet a tu quoque not with complaints of whataboutism but with a calm, “Yes, I was wrong, and here’s how I’ve changed.” And then get the conversation back to the topic at hand.

Which ought to be, not “Who’s worse?” but “How can we make society better?” This will not be accomplished by deflecting accusations into an inquiry into the behavior of the accuser — but nor will it be accomplished by allowing flagrant hypocrisy to pass unremarked. Allowing brazen hypocrites to demand social sanction (but only for others) does not uphold important principles; it destroys them. Both sides come to view principles merely as useful weapons — and soon, they find that they are not useful even as weapons. As economist Garett Jones recently told me, “Two wrongs don’t make a right . . . but three wrongs make a social norm.” And “Do as I say, not as I do” is not a principle that any decent society should endorse.

Megan McArdle is a Bloomberg View columnist. She wrote for the Daily Beast, Newsweek, the Atlantic and the Economist and founded the blog Asymmetrical Information. She is the author of “The Up Side of Down: Why Failing Well Is the Key to Success.”