Washington • After the second two-night Democratic candidates’ “debate,” Joe Biden remains a vulnerable front-runner. And it’s definitely time to start winnowing the field.

I put the word debate in quotes because this week’s encounter, hosted by CNN, was structured to maximize conflict and minimize clarity. In a typical exchange, one of the moderators — network stalwarts Jake Tapper, Dana Bash and Don Lemon — would say something like, “Candidate X, your rival Candidate Y says you have poopy pants. How do you respond?” Candidate X would reply that Candidate Y’s mother has a moustache. Then they would go back and forth until Candidate Z chimed in with a touching story about meeting a mustachioed mother on the campaign trail who struggles with medical costs.

I'm being slightly unfair to CNN, but only slightly. I accept that there's no way to have a truly satisfying exchange of views with so many candidates onstage. But orchestrating a series of gladiatorial battles was a poor choice.

That said, the big question was whether Biden — still far ahead in the polls — could rebound from a disastrous performance in June, when he seemed tentative, confused, almost doddering. The Biden we saw Wednesday night was different, or at least different enough. He mixed up his numbers a couple of times, saying "3 trillion" when he meant "30 trillion," and his final words of the night — an apparent attempt to tell viewers how to send a text to his campaign — were incomprehensible. But during thrust-and-parry exchanges with his rivals, Biden generally gave as well as he got.

He could have been better, though. Biden has an all-purpose shield that he can use to counter any attack or bolster any argument he's trying to make — his eight-year service as vice president under the most revered figure in the party, Barack Obama, who is viewed favorably by 97% of Democrats.

So when the candidates favoring various versions of Medicare-for-All accused Biden of incrementalism, he could say he was defending Obamacare. When Biden's role in the legislation that set the stage for mass incarceration or his views on race and gender came up, he could say that he was righteous enough for Obama's tastes. Biden indeed did wrap himself in the Obama mantle several times, but he seemed awfully slow to do so.

The contender who metaphorically knifed Biden in the June debate, Sen. Kamala Harris, was less effective this time around. She flubbed a couple of her lines, and her attempt to go after Biden for flip-flopping on the Hyde Amendment — which bans the use of federal funds to pay for abortions — was little more than a glancing blow.

Sen. Cory Booker had a much better night, tearing into Biden for his major role in the 1994 crime bill — to which Biden responded by recalling Booker's tough-on-crime policies when he was mayor of Newark, New Jersey, which included stop-and-frisk policing. I'd give that round to Booker, but I don't know if it was enough to lift him out of single digits in the polls.

Sen. Kirsten Gillibrand's attempt to use a nearly 40-year-old op-ed to paint Biden as some kind of Neanderthal who believes a woman's place is in the home was, frankly, pathetic.

On Tuesday night, in the first of the debate's two sessions, Sen. Elizabeth Warren showed why she has been steadily moving up in the polls. She strikes me as the best debater of the lot. She has great command of the facts and figures she uses to bolster her arguments, and she has the ability to make her scripted applause lines sound spontaneous.

She needed all those skills and more, as she and Sen. Bernie Sanders came under withering attack from moderates such as Reps. John Delaney and Tim Ryan for, in their view, dragging the party disastrously far to the left. Some commentators had predicted that Sanders and Warren would fight each other for leadership of the party's progressive wing. Instead, they joined forces to defend their shared vision of a Medicare-for-All health system. Sanders, even more than usual, sounded all night as if he were yelling out the window at kids making a racket.

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Folks: Whether to achieve fully universal health care through a pure single-payer system or an expanded version of Obamacare is an interesting theoretical discussion. Neither is remotely possible until a Democrat replaces President Trump and another replaces Senate Majority Leader Mitch McConnell.

Some of you candidates need to focus on Trump. Some of you others need to go home and run for the Senate.

Eugene Robinson