Growing up in Detroit, I was taught that two wrongs don’t make a right. I also was taught that two wrongs are the right approach to a bully. Detroit is a complicated place.

As my father explained, a bully is an insecure person overcompensating for something. Find that weakness, ridicule it, and defuse the bully’s bombast with a dismissive laugh. “Bully the bully,” Dad taught his four kids. “And you might not have to fight him.”

Which brings me to Donald Trump. He is vain. He is weak. He is overcompensating for something. He calls women “disgusting animals”—fat pigs, dogs, slobs, and bimbos. He calls Mexicans rapists. He insults any journalist who dares to question him. He wants to shutter mosques and ban Muslim immigration.

These attacks follow a familiar pattern: Their targets represent a threat to Trump’s vainglorious self-image.

For far too long, the Republican establishment and Trump’s GOP presidential rivals didn’t take his candidacy seriously. Even after recognizing that Trump could win the nomination, the members of the feckless GOP leadership flinched. They didn’t want to be Trump’s next targets.

Sure, Jeb Bush attacked Trump, even belittled him a bit, but the candidate’s Right to Rise super PAC foolishly failed to pile on. Said a cocky Mike Murphy: “Trump is, frankly, other people’s problem.”

Now he’s everybody’s problem—and, finally, somebody is doing something about it.

After Trump won three straight contests and looked to claim the nomination on Super Tuesday, Florida Senator Marco Rubio decided to fight slime with slime. He taunted Trump over his made-in-China ties and his spray tan. He mocked Trump’s spelling. He called Trump a pants-wetter. He questioned Trump’s manhood.

“You know what they say about men with small hands,” Rubio smirked, “you can’t trust 'em!” Rubio knows that’s not what they say about men with small hands.

On the one hand (pun intended), Rubio’s barbs were a disappointment. Politics needs more, not less, civility. Twenty-first century culture is coarsened enough without America’s leaders setting a bad example on the campaign trail. But if you believe, like I do, that Trump is fueling and exploiting people’s prejudices to seize power—that he’s a bully seeking the presidential pulpit—then this feels like a long time coming.

It may be too late to prevent Trump from winning the GOP nomination, but Rubio is showing Democrats how to fuddle him. Rubio has crawled beneath Trump’s skin and is gnawing his way out.

In just the last few days, Trump:

  • Threatens to curb the media’s right to criticism him.
  • Retweeted a Benito Mussolini quote.
  • Refused to denounce David Duke and the Ku Klux Klan.

“I don’t know anything about David Duke,” he told CNN’s Jake Tapper. The man who claims to have “the world’s greatest memory” seems to have forgotten that just two days earlier he disavowed the former Klansman, if only grudgingly. “I disavow, OK?” It must also have slipped Trump’s mind that he refused to run for president on the Reform Party ticket because Duke was a member of the party.

Suddenly, Trump finds himself in a defensive crouch. Republicans are standing up to him. “A brokered convention is now just the fantasy of Republican elites and Marco Rubio fans,” former Republican Representative Joe Scarborough, host of MSNBC’s “Morning Joe,” wrote in the Washington Post. “The harsher reality is that the next GOP nominee will be a man who refused to condemn the Ku Klux Klan and one of its most infamous Grand Wizards when telling the ugly truth wouldn’t have cost him a single vote. So is this how the party of Abraham Lincoln dies?”

Rubio says he had hoped to lead a campaign of ideas. But, as The New York Times reported, he found himself in a reality-TV show.

As they watched Mr. Trump clinch his third straight victory with a win in the Nevada caucuses last week, Mr. Rubio and his aides concluded that the only way to beat him was to get inside his head, by stooping to his level: Taunt, insult, mock and have a blast doing it.

They seem satisfied that it is working.

“We came to the conclusion that if being a part of the circus is the price you have to pay in order for us to ultimately be able to talk about substantive policy, then that’s what we’re going to do,” said Todd Harris, a senior Rubio adviser.

Mr. Harris noted that Mr. Rubio’s speeches were now being carried live on television. And if the price of admission, he added, was talking about “how Trump is a con man, with a bad spray tan,” so be it.

What has followed is a race that looks more like a variety show than a campaign to elect the most powerful leader on earth.

There is the awful truth: Trump is not merely the fault of the GOP; he is a reflection of an entire political system, including political journalism. When the political establishment puts polls above people, ratings above accountability, celebrity above credibility, exploiters like Trump prosper.

Now, belatedly, people are starting to bully the bully. It may soon be time to fight him.