Every front-runner can benefit from having an attack dog on his side, savaging his rivals while he stays a little more above the fray. In 2008, John McCain slipped through to the Republican nomination in no small part because Mike Huckabee just hated Mitt Romney, and stuck knives in Romney every chance he got. In 2016, Donald Trump won the New Hampshire primary easily because Chris Christie, an also-ran by then but also Trump’s de facto ally in the “Tri-State-Area bully” lane, savaged Marco Rubio so thoroughly that the usually smooth Floridian essentially glitched onstage during a debate, seemingly confirming every anti-Rubio critique.

In the same way, Biden benefits from the moderates-versus-Sanders and moderates-versus-Warren scenes that played out repeatedly on last night’s stage, because they help him escape the essential dilemma of his candidacy: That he can probably win if he consolidates and holds the moderate wing of the party — but that the work of consolidation, to the extent that it requires frontal attacks on the woke and neo-socialist styles of liberal politics, could threaten party unity even more than the Sanders-versus-Hillary Clinton conflict four years ago.

Imagine Biden fighting Warren and Sanders and (depending on her positioning) Kamala Harris the way Delaney did last night, on the way to a narrow victory at a contested convention. Team Biden would surely take the victory, but it’s not at all the scenario that his campaign (or any Democrat) should want. And that scenario assumes that Biden is even capable of taking the fight to the more left-wing candidates in a sustained and compelling way. On the evidence of his first debate performance, he might need an attack dog because he himself simply isn’t quick or supple enough for onstage combat anymore.

We’ll know more after tonight’s debate, which will probably test him not only with whatever Harris has prepared, but also in the same way that Warren and Sanders were tested, with Bill de Blasio filling the John Delaney role and attacking Biden from the furthest left at every opportunity. In which case Biden’s tricky task, unless he gets unexpected backup, will be to prove he can counterpunch without seeming to be too anti-left, too ready for ideological civil war.

But having emphasized all the pitfalls ahead for him, it’s worth returning to where we started: With Biden having a good night while his leftward rivals fought the moderates. Yes, come the fall he won’t have Delaney or Hickenlooper to fight his battles for him. Yes, he has to prove that he can marry toughness and finesse. Yes, his obvious path to the nomination involves a lot of intraparty strife. But he’s holding 30 percent of the primary electorate when nobody else has much more than half that, his core constituency is precisely the sort of voter who doesn’t give a rip who left-wing or media Twitter thinks is winning the debates, and three weeks after Harris supposedly demolished him, their whole encounter barely even registers in polls.