Photo: Eric Thayer for The New York Times; Video: By CNN

Few criticisms cut at Senator Ted Cruz like being called a liar. It undermines the central tenet of his candidacy, that he is a trusted conservative, his principles inviolable.

So when Donald J. Trump did just that during the debate — “This guy is a liar,” he said, meaning Mr. Cruz — the Texas senator was outraged.

The moderator, Wolf Blitzer, was ready to move on from an exchange that was already descending into bitterness and acrimony. “You’re going to say that I can’t respond to being called a liar?” Mr. Cruz demanded. Mr. Blitzer let him finish.

Mr. Cruz’s response? Mr. Trump is the real liar.

“Let me tell you something,” Mr. Cruz said. “Falsely accusing someone of lying is itself a lie, and it’s something Donald does daily.”

Whatever civility might have remained in that exchange eroded from there, and it ended up as a barely intelligible shouting match. And no matter which candidate may have gotten the better of the other, it was difficult to see how anyone gained anything from it.