After the Democratic primary debate on Thursday, top-shelf political journalists were quick to declare Joe Biden the winner by default.

Biden turned in "a solid but unspectacular showing" that was good enough "for the former vice president to win the Democratic nomination and maybe even the White House," wrote Stephen Collison at CNN.

"Joe Biden on Thursday delivered the kind of performance his supporters have been waiting for — combative when needed and in the thick of the action throughout," wrote The Washington Post's Dan Balz.

Biden "fights off rivals," wrote Katie Schubauer and Michael Mathes at AFP.

"Biden won, again," wrote Jonathan Last at The Bulwark.

I have only one question: Were these folks watching the same debate as I was?

Because while nobody quite executed a Chris Christie-style suicide attack on Biden, his performance was still at-times gobsmacking evidence of a man whose mental acuity is fading by the day.

Julian Castro was the only candidate to take a serious run at Biden's biggest weakness. They got in an argument over health care — Castro said that his plan would automatically enroll anyone who lost their insurance, while Biden's would make people "buy in." When Biden insisted that wasn't true Castro cut in: "Are you forgetting what you said two minutes ago?" It was a clear dig at Biden's decaying mind, and it elicited shock from the audience.

Now, fact checkers noted that Biden was arguably correct about his plan — though Castro was correct that Biden said people would buy in. ("If you want Medicare, if you lose the job from your insurance — from your employer, you automatically can buy into this," he said.) As Libby Watson writes, even if Biden ends up being right about what's on his website, it's not exactly comforting if he can't seem to keep the details of his plan straight in his head.

But by far the most memorable moment of the debate came when moderator Linsey Davis — perhaps inspired by The New York Times' recent 1619 Project — asked Biden about scornfully dismissing any responsibility to correct the lingering harms of slavery years ago. He responded with a rambling tirade about how black parents don't know how to raise their kids:

We bring social workers into homes of parents to help them deal with how to raise their children. It's not that they don't want to help. They don't know what — They don't know what quite what to do. Play the radio. Make sure the television — excuse me, make sure you have the record player on at night. The phone — make sure the kids hear words. A kid coming from a very poor school — er, a very poor background will hear 4 million words fewer spoken by the time they get there. [Slate]

The "words" comment appears to be reference to a much-disputed 1995 paper about poorer kids not hearing as many words as wealthy ones (a result which failed to be replicated in a recent study). The "record player" comment might be a reference to the goofy urban legend that playing Mozart while your baby is asleep will make them a Super Genius. It's a stupid argument, delivered in alarmingly incoherent syntax — and moreover, as Anand Giridharadas points out, given Biden's appalling record of teaming up with outright segregationists to roll back the Civil Rights Movement, the substance of his point here is extremely offensive.

Linsey then attempted to cut him off. But Biden plowed ahead, changing the subject randomly to … Venezuela and Nicolás Maduro?

No, I'm going to go like the rest of them do, twice over, okay? Because here's the deal. The deal is that we've got this a little backwards. And by the way, in Venezuela, we should be allowing people to come here from Venezuela. I know Maduro. I've confronted Maduro. No. 2, you talk about the need to do something in Latin America. I'm the guy that came up with $740 million to see to it those three countries, in fact, change their system so people don't have to chance to leave. You're all acting like we just discovered this yesterday! [Slate]

What in God's name is he talking about? Who has what backwards? We've got to let Venezuelans come here and also spend some money to change those "three countries" so people "don't have to chance to leave"? What?

Anyway, centrist Democrats and Biden-friendly media elites like Joe Scarborough had an immediate hissy fit over Castro being a big meanie. That will probably keep anyone from pelting Biden with bottles of Ensure at the next Democratic debate. But as The Washington Post's Greg Sargent argues, the Trump campaign is not going to be so generous. On the contrary, they will level a "massive onslaught of absolutely brutal and distortive attacks" on Biden over this — likely including doctored video and audio.

Now, Trump would do duplicitous attacks on any opponent. But the effectiveness of agitprop depends in part on whether the target has an actual weakness — the best lies have a kernel of truth in them. And Biden quite clearly is massively vulnerable on whether he is mentally acute enough to be president. Journalists trying to paper over that fact are doing Democratic voters no favors.