The Republican debate on CNBC was riveting, the way a train wreck is riveting — you can’t take your eyes off it. The Fox Business Network debate was merely satisfying. A serious political discussion requires a bit more work, but it repays the effort.

The CNBC affair was a contrived food fight during which substance occasionally broke out (such as the brief exchange between Chris Christie and Mike Huckabee on entitlement reform). FBN, on the other hand, conducted a meaty debate during which a tomato or two was occasionally tossed. John Kasich came itching for a fight. Donald Trump pitched back with his usual high-mindedness, responding at one point to Kasich with “I’ve built an unbelievable company worth billions and billions of dollars. I don’t have to hear from this man.”

Despite such exceptions, the FBN debate marks the point at which the GOP campaign begins leaving the entertainment phase and entering the serious season. The moderators’ modesty and straightforwardness created an atmosphere of transparency that allowed the candidates to reveal themselves, advertently or not.

Kasich did. Unfortunately, it was an irritable self-righteous Kasich who showed up, doing himself no good. At the other end of the podium, Rand Paul had his best night. He certainly deserves credit for courage. His noninterventionist foreign policy is far outside the GOP mainstream, which is why Marco Rubio won the room in their exchange on defense spending and intervention. But Paul defended his minority view stoutly, regardless. Give him points for principle.

In a year when showmanship is king, however, principle won’t help him much to get out of single digits. You could almost see Paul on the far right of the stage and Kasich on the far left dropping through a trapdoor, leaving six finalists.

Or perhaps not six. Jeb Bush, too, had his best night. He was competent and solid but, unfortunately, still inarticulate. You almost feel sorry for the travail he is about to endure on his increasingly long-shot campaign.

Carly Fiorina, strong on stage but weak on campaign infrastructure, showed herself tough as nails — the perfect VP. She can say things about Hillary Clinton that no man can. And she knows it.

Tuesday’s best performers, unsurprisingly, were Rubio and Ted Cruz, the 44-year-old, silver-tongued, Cuban-American, first-term senators. Imagine them as a ticket. In 1992, Bill Clinton’s choice of Al Gore was as strategically brilliant as it was counterintuitive. Instead of balancing that ticket — old with young, Northern with Southern, experience with energy — Clinton doubled down with his own mirror image. The “Young Guns,” as Newsweek memorably dubbed them on its cover, proved irresistible. (Others called it “Double Bubba.”) 

Which leaves the two outsider front-runners. Ben Carson had an awful night — the Chinese intervening in Syria? But it was bookended and thereby saved by two good moments: his first answer, the pre-emptive “Thank you for not asking me what I said in the 10th grade,” and his closing statement about the suffering in the country being overcome by America’s inner strength.

Trump shares with Carson the GOP’s vast anti-politics constituency, now fully half of the Republican electorate. Carson’s antidote to the nation’s failed politics is moral strength. Trump’s is unapologetic brute strength.

Trump did not have a particularly good night, either. He was again at sea on foreign policy. And when asked about the Trans-Pacific Partnership, the 12-nation trade deal he opposes root and branch, Trump did his riff on the Chinese economic menace — to which Rand Paul calmly pointed out that China is not party to the TPP. Indeed, the main strategic purpose of the TPP is precisely to contain China by binding its Pacific neighbors to the United States, thus blunting Beijing’s reach for regional hegemony.

Never mind. As long as the anti-politics mood prevails, neither Trump nor Carson is even dented by such policy misadventures.

Tuesday night did not radically alter the trajectory of the Republican race. But it will hasten the winnowing of the field. If you narrow the viewfinder, the debate stage shrinks from eight to six to a possible final four: Cruz, Rubio, Carson, Trump. (Chris Christie, who shone in the undercard debate, has the best outside shot at crashing this group.)

On Tuesday, all the contenders were required to show their hand. We saw character and we saw policy. Substance is never sizzling, but the FBN debate was both revealing and sobering: Which one of these can you actually see inhabiting the Oval Office?

© 2015, The Washington Post Writers Group