Donald Trump is a master troller, and so far, it’s worked out for him. But there is a sense that Trump may have reached his ceiling, and—not coincidentally—this seems to have coincided with him running into a couple of candidates who couldn’t be bullied.

Remember, there was a time when attacking Trump was not only a fool’s errand, but also an invitation to sink in the polls. Rick Perry and Bobby Jindal—both serious governors—tried, and failed, to put him in his place. Rand Paul was likewise unsuccessful. When you pull a knife, Mr. Trump pulls a gun in the form of a stream of Twitter and TV insults.

Trump likes to say that he only counterpunches, but he seized on comments made by one of Scott Walker’s fundraisers to declare “fair game” on the union buster. Last time I checked, Trump was still here and Walker was long gone.

The first time Trump lost a game of one-upmanship was when Carly Fiorina confronted him in that CNN debate. Trump had insulted her “face” in the past, but found himself on the wrong end of a smackdown. This happened because Fiorina is tough and fast on her feet. And, of course, because she’s a woman—and, in this regard, sui generis in the GOP presidential field—she was able to seize the moral high ground.

Something happens the first time a bully gets beaten: The sense of omnipotence and inevitability is gone. When pundits and commentators argue that the CNN debate was a turning point for Trump (despite the fact that his polls have held pretty steady), this is what they’re talking about.

Something else happened: Trump’s politics, you have to understand, are somewhat primitive. Much of his appeal is based on being an Übermensch—someone who is a winner and can will victory—by force of his personality. His putdowns essentially suggest that he’s the alpha male. Thus, the pretense of a serious political debate has become something of a game of machismo.

This is not entirely new—what was Al Gore doing when he stood up, walked toward his opponent, and tried to intimidate George W. Bush during that 2000 debate if not exert his status? But Trump has removed what was left of the façade about politics being a gentlemanly sort of thing. Chivalry is dead, and Trump’s here to prove it.

The problem, of course, is that his game of being manlier falls apart when he faces a female (keep in mind that Hillary is the most likely opponent). It backfires. He either looks mean—or he is the one who is emasculated. In the case of the CNN debate, both things, to some degree, happened.

Perhaps deciding Fiorina isn’t a good target, Trump has set his sights on Marco Rubio (in fairness, the two men have been feuding, of late). This makes a certain amount of sense. If there are two “brackets”—an “insider” bracket and an “outsider” bracket—then one can assume that it’s plausible Rubio and Trump will be the last two men standing in their respective division.

Just the other day, Trump trolled Rubio by sending him a care package of water bottles and towels. This was because Trump says Rubio is prone to “sweating.” But here’s the thing: Rubio seems to be enjoying this. And I don’t think he’s faking it. I think he actually enjoys this gag gift.

Trump seems to get under the skin of some candidates, but not Rubio. And I have a theory as to why. It goes back to something I read about Lee Atwater, the infamous strategist who helped George H.W. Bush defeat Bob Dole in 1988. Atwater was a master at psychological warfare. As author John Brady reconuts in his book, Bad Boy: The Life and Politics of Lee Atwater, he believed that “adults could be divided into two groups: the overly mature and the childlike.”

“The overly mature,” Brady writes, “are inflexible and overserious, making them highly vulnerable in politics, particularly in the age of television. [Bob] Dole was the mature type, Atwater the child.”

My theory is that Rubio still has enough of a childlike quality—I’m not talking about childishness, but a sort of “neoteny” whereby he has tempered his thoughtful conservative side with a playful or youthful, devil-may-care side—to make him actually enjoy Trump’s juvenile antics.

Back to Atwater, Brady writes: “It didn’t take Atwater much research to see that Dole was hypersensitive about attacks on his wife. Replaying old charges against her in Iowa, Atwater was able to get under the senators’ skin.”

Guess who else is sensitive about attacks on his wife? That’s right; remember Trump’s attack on Jeb Bush’s wife—and his failure to apologize for it at the debate?

I doubt Trump has studied exactly how Atwater and Jeb’s dad handled Dole. More likely, juvenile button-pushers like Atwater and Trump intuitively recognize who they can victimize—and how. Bob Dole, of course, ended up looking sour, famously declaring that George H.W. Bush should “stop lying about my record.” No such luck with Fiorina or Rubio.

The problem for Donald Trump might be that he’s going to run out of Bob Doles.