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 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.

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.

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 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. Sanders, even more than usual, sounded all night as if he were yelling out the window at kids making a racket.

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's email address is eugenerobinson@washpost.com.