We can’t even compare this cat to Hitler, and look at it! (Paul Neve/catsthatlooklikehitler.com/FTWP)

You can fool all the people some of the time and some of the people all of the time.

You can do almost anything you want in this world, even things that many people find deeply objectionable. You can take an elaborate dump on a religious object and call it art. You can send text messages on the toilet. You can have carnal relations with a goat and afterwards insist that the goat was a woman when you started. (This has yet to work, but my point is, you can do it.)

But just try comparing anyone to Hitler.

Especially when running for office.

This is one of the trade-offs we make in public life. There are certain behaviors that are commonly understood to mean one thing only. Put your napkin on the table, and you have finished eating dinner. Hit the turn signal, and you are about to change directions. Tell the media that you have no interest in the vice presidency, and you are tossing your hat in the ring. Compare someone to Hitler, and you are admitting that you have lost the argument.

Perhaps Dr. Marisha Agana, running for Congress in Ohio, did not realize this when she tweeted Sunday, “History has a way of repeating itself: Stalin, Hitler, Mao Tse Tung and now Obama!!!” The backlash was instant. She half-apologized for it on Monday, noting, “I am sorry if I made a poor choice of analogy but I was referring to his pro-abortion stance!” and posting a more detailed explanation of her views on Facebook. But the damage had been done.

Not just Hitler, but Stalin too?

The Hitler comparison, traditionally, is an indication that you have run out of argument. Even if you haven’t. You may well have bucketloads of sound reasoning still sitting by waiting to be deployed, myriad statements both beautifully expressed and profound. But the Hitler comparison is the universally acknowledged signal for Rational Dispassionate Discussion to glance at its watch and announce that it has somewhere else to be.

“Look out!” the Hitler comparison says. “I just glanced into my store of talking points and the cupboard was bare. Expect me to conduct the rest of this discussion in ALL CAPS.”

And no amount of temporizing afterwards can soften it. “Oh no,” you say, “I was just likening him to Hitler because both of them have mustaches.” “Oh no,” you exclaim, “I just wanted to draw the comparison because neither of them has ever offered to paint my house.” ”Oh no,” you point out, “I just meant that they both have last names of two syllables.”

Sorry. The damage has been done. We knew what you meant the first time.

Note: For the canonical discussion of Hitler comparisons in online discussion, see Godwin’s Law.