For nearly three hours, the debate remained within the tinny echo chamber of the Republican Party. Photograph by Chris Carlson/AP

With about fifteen minutes to go in the G.O.P. Presidential debate last night, at the Ronald Reagan Presidential Library, it looked as if it might be hard to pick a low point. The candidates had, after all, been squabbling for almost three hours, long enough to foster fantasies of using Reagan’s Air Force One, which was onstage, as an emergency-escape vehicle. Then Jake Tapper, the moderator, asked Ben Carson about vaccines—or, more specifically, what he, as a pediatric neurosurgeon, thought of Donald Trump's statements linking childhood vaccines to autism, "which, as you know, the medical community adamantly disputes." Perhaps Tapper thought that he was setting up a confrontation between rationality and Trumpism, a moment when the doctor would calmly dismantle the Donald. If so, he was disappointed. Carson, after allowing that studies "have not demonstrated that there is any correlation between vaccinations and autism," and that some inoculations saved lives, added that "a multitude" of them probably weren't important—“and there should be some discretion in those cases. But, you know, a lot of this is—is—is pushed by big government.”

"Should he stop saying that vaccines cause autism?" Tapper asked.

Trump, Carson replied, was "an intelligent man and will make the correct decision after getting the real facts."

Trump proceeded to demonstrate his decision-making qualities with a digressive rant about how he knew "a beautiful child" who "went to have the vaccine, and came back, and a week later got a tremendous fever, got very, very sick, now is autistic." He'd had his own regimen for his children, he said, spreading things out; if people followed his advice, "I think you're going to see a big impact on autism."

You're not: the science is very clear that vaccines do not cause autism, and that all the deniers have accomplished is to provoke outbreaks of diseases, like measles, that were once almost forgotten in America. (My colleague Michael Specter has written about this extensively.) A lot of what Trump says—diplomacy by yelling, for example—would be dangerous if put into practice. But most of it, assuming he doesn't actually get elected, won't be put into practice. The refusal to inoculate children, though, is something that his admirers can try at home. No other candidate was willing to anger the ideologues by standing up for something as suspicious as science. Given a final chance by Tapper, Carson smiled and said that Trump was "an O.K. doctor"—a reference to Trump's dismissal of Carson’s surgical skills. Rand Paul, who is also a doctor, added that though he was for vaccines, "I'm also for freedom"—namely the freedom to reschedule shots, "even if science doesn't say" that there's a problem.

The exchange was of a piece with the rest of the debate and with the state of the Republican Party: fervid, claustrophobic, recklessly insinuating, and, at the same time, utterly timid when it comes to extremism in its own ranks. The discussion about vaccines was immediately preceded by one about climate change: Tapper—citing George Shultz, Reagan's Secretary of State, who said that his boss had urged industry leaders to come up with a plan to move away from chemicals destroying the ozone layer "as an insurance policy in case the scientists are right"—asked Marco Rubio why we shouldn't now do the same, just in case there was something to the overwhelming evidence of climate change, rising sea levels, and retreating glaciers all around us.

"Because we're not going to destroy our economy the way the left-wing government that we are under now wants to do," Rubio said. "Single parents already struggling," he added, couldn't afford to do things that, in his view, wouldn't affect the climate. And anyway, "America is not a planet."

Tapper turned to Chris Christie, apparently on the theory that he had previously been more willing than other Republicans to recognize the reality of climate change. He didn't have much better luck than he did with Carson. "I agree with Marco. We shouldn't be destroying our economy in order to chase some wild left-wing idea that somehow us by ourselves is going to fix the climate," Christie said. Not that he was interested in working with others: he bragged that, while making the air cleaner in New Jersey, he'd pulled the state out of regional cap-and-trade agreements. America may not be a planet, but New Jersey is. Scott Walker then jumped in to say that he wouldn't listen to the E.P.A. Tapper managed to move on, despite Paul’s clamoring to be heard "if you want a skeptic" about climate change. No one tried to break in on behalf of the planet—the one that America is on.

The flaw in the debate, evident in both of these exchanges, may have been a structural one. In the interests of mixing it up, CNN's moderators—Tapper was joined by Dana Bash, the network's political correspondent, and Hugh Hewitt, the conservative radio host—repeatedly quoted criticisms that one candidate had made of the others. The most blunt example may have been when Carly Fiorina was asked what she thought of Trump's comments, to Rolling Stone, about the unelectability of her face. In lieu of an apology last night, Trump said, "she's a beautiful woman." (He also said that he’d heard that Jeb Bush’s wife was “lovely.”) Fiorina declined to simper; throughout, she played her designated role of Trump teller-offer, mixing in references to "my good friend Bibi Netanyahu" and lurid (and inaccurate) descriptions of secretly taped videos of Planned Parenthood employees. Later, she and Trump sniped about their respective business records, prompting Christie, of all people, to tell them to "stop playing" and being so "childish." "And Carly—Carly, listen," he said. "You can interrupt everybody else on this stage, you're not going to interrupt me, O.K.?" He sounded like a first grader striking a pose for the kindergartners.

But rather than really pushing the candidates on their positions, CNN's did-you-hear-what-he-called-you approach served to keep the debate within the tinny echo chamber of the G.O.P. For example, Bash asked Jeb Bush, "After Chief Justice John Roberts voted to uphold Obamacare twice, Senator Cruz criticized your brother for appointing John Roberts to the Supreme Court. Looking back on it, did your brother make a mistake?" What followed was a somewhat bizarre exchange in which Bush offered a tepid defense of Roberts while saying that Cruz had supported him, too. Cruz replied by comparing Roberts—who, Obamacare aside, has led the court to achingly conservative decisions in Citizens United and Hobby Lobby, has helped to roll back voting rights, and has written bitter dissents on marriage equality—to the far more liberal David Souter. Mike Huckabee, who had earlier said that Kim Davis, the county clerk in Kentucky who is violating the Supreme Court's Obergefell decision by refusing to issue marriage licenses to same-sex couples, had been afforded less respect for her religious beliefs than "the detainees at Gitmo," jumped in to expostulate about litmus tests. Only in the Republican primaries would any of this make sense.

The Roberts exchange was one of several in which Jeb Bush couldn't shake a certain air of petulance. "That's—that's my brother," he said at one point, wiggling his arms a little. He had been trying to finish an evocative line about George W. Bush embracing a firefighter in the rubble of Ground Zero, but, since he'd been interrupted with interjections from, in sequence, Trump, Paul, and Walker, it was hard to figure out just what he was saying, or even parse it grammatically. He certainly didn’t command the foreign-policy discussion, amid a general clamor about renouncing the Iran agreement and Trump dismissing the need to learn “name after name, Arab name, Arab name.” Both Rubio and Carson had stronger moments in explaining why they might, if only occasionally, think before bombing, but both still came across as flagrantly hawkish. Paul, meanwhile, communicated the anti-interventionism that has set him apart. (His most effective moment, though, came when he talked about decriminalizing drug offenses, prompting Bush to talk about smoking marijuana as a teen-ager and Fiorina to speak, movingly, about her step-daughter who lost her life to addiction.) Bush said that he had chosen the same foreign-policy advisers that his brother and father had because, if you wanted a Republican, those were the only people available, “just by definition.” Near the end of the debate, Tapper asked each candidate to pick his or her Secret Service code name. (Paul: "Justice Never Sleeps"; Cruz: "Cohiba"; Walker: "Harley. I love riding Harley's.") Bush went with "Eveready—it's very high energy, Donald." Suddenly, it became hard to shake the image of Jeb as a giant Energizer Bunny. It also made him sound like a sulker, fixated on Trump's description of him as "low-energy." Trump, for his part, offered Eveready a hand-slap and then proposed his own name: "Humble."