I am obviously a partisan for Sanders, but six months ago if you asked me who could beat Trump I would give you two names: Bernie Sanders and Joe Biden. Everyone else has extremely shaky numbers against Trump, and some candidates in particular (Warren and Buttigieg) seem personally ill-matched for his combative reactionary populism. Biden, meanwhile, has polled well against Trump, which is why my argument against him was entirely political: I saw no reason to expect him to lose.

So when I say that I’ve changed my mind, please for the love of god do not read this as a cynical argument for Sanders. I was not saying this six months ago. Even now I don’t have to say it for Sanders, because he doesn’t need it. The case for Sanders is still that he is the best candidate who can beat Trump, and this is true even if worse candidates (like Biden) can also beat Trump.

But I don’t think Biden can. Not anymore.

Biden’s behavior in this primary has been erratic and bizarre. This is not just the goofy, gaffe-prone Biden we remember from the years before his retirement — that Biden was undisciplined, but he was diplomatic and sharp. This Biden is unpredictable, often confused, and occasionally flat-out disturbing. When he speaks he spins his wheels, meanders onto bizarre tangents, and stumbles over simple points of fact. When he interacts with people, he veers from uncomfortably familiar to wildly aggressive.

Just look at this video. Joe Biden:

Calls a voter a liar

Flirts with calling him fat

Challenges him to feats of strength

Challenges him to an IQ test

And makes fun of his age

This is completely unhinged. More importantly, this will not win votes. People will suspect that Biden is ridiculous, or cruel, or creepy, or dim-witted, or unwell, or simply a bad politician — and some, inevitably, will decide that they just can’t vote for him.

Biden’s behavior is especially damaging in a race against Trump for two reasons:

Biden is going to run as a return to normalcy and respectability from the embarrassing aberration of Trump. Most other candidates (including Clinton, by the way) could pull this off. The Biden we remember from a decade ago could pull this off. But the Biden we are seeing today is going to leave voters wondering if they are just trading one embarrassing weirdo for another. Trump is going to say that Biden is senile and unfit for office. He is not going to just imply it or let attack dogs say it like other Republicans might — he is going to say this explicitly and repeatedly and make it a major part of his campaign. He is not going to care that one might say this about him because he has the shamelessness and audacity of a sociopath. He is going to be cruel and bullying about it, and he will have the benefit of low expectations because no one expects anything else from him. This will be especially brutal in debates, because for all of his faults, Trump is still very quick on his feet verbally, and is very conscious of this advantage.

For the first several months of his campaign, Joe Biden did what all front-runners do: he attempted to stay above the fray of the primaries, limiting his public appearances and coasting on his reputation. It is only in recent months, first in debates and then during his forays into early-state retail politics, that he gained significant public scrutiny. That’s why it has been extremely easy to miss the change: we began the year with memories of the old Joe Biden, and have only seen the new one emerge quite gradually.

But I am telling you now, this is not a Joe Biden who can beat Donald Trump. The Democratic establishment sees this, which is why they’ve hedged their bets on so many other candidates. The public sees this, which is why Biden has struggled with donations. And I promise you that the media sees it too, even though they are not saying it out loud. If you want to beat Donald Trump, there is one safe bet — one candidate that we need to rally behind yesterday. And it’s not Joe Biden.