Julián Castro and Cory Booker went there on Joe Biden on Thursday night. Neither of them said it — that Biden is too old — but they might as well have.

Castro at one point wrongly accused Biden of forgetting something he had just said on the debate stage, animatedly repeating, “Are you forgetting what you said two minutes ago?"

Then Booker appeared on CNN after the debate and had this to say:

BOOKER: I think that we are at a tough point right now because there’s a lot of people who are concerned about Joe Biden’s ability to carry the ball all the way across the end line without fumbling. And I think that Castro had some really legitimate concerns about can he be someone in a long grueling campaign that can get the ball over the line and has every right to call out.

Q: Do you think that Biden did a better job tonight? Do you think that he showed that he could take the ball over the line? People are saying, look, he was stronger.

BOOKER: I think there were a lot of moments where a number of us were looking on the stage when he tends to go on sometimes. At one point, he was talking about communities like mine listening to record players. I don’t remember the last time I saw a record player. ... But there are definitely moments where you listen to Joe Biden and you just wonder. But I don’t know —

Q: Senator, are you saying that he’s just too old to be president?

BOOKER: No, I’m definitely not saying that because I’ve listened to Joe Biden over the years and often felt like there were times that he is going on or meandering in his speech. Look, I want someone that can excite and energize and call us to a campaign like we saw back in ’08, in ’12 where we saw record turnouts and somebody who can speak to the fullness of the Democratic Party. If I believed Joe Biden was that person, I wouldn’t be sitting here.

We can debate all day whether these are veiled or not-so-veiled attacks on Biden’s age. (Castro claimed after the debate he was just engaging on policy.) But that’s kind of beside the point. Two of the candidates on the debate stage Thursday night accused the candidate who leads almost every Democratic primary poll of lacking both mental and verbal acuity. Doing so certainly highlights Biden’s age, which is an unavoidable topic given he would by far be our oldest president and certainly has had his share of flubs and senior moments. Even if that wasn’t the intent, the effect would be similar: It calls into question his fitness to be president.

There are many things that can be papered over in a general election, as we saw in 2016 when Republican presidential candidates called Donald Trump all manner of things from “crazy” to “bigoted” to “nuts” to “sniveling coward” to “dangerous.” We’ve written repeatedly about how ridiculous it is for Republicans to say these things and then pretend like they didn’t really mean them and/or that it was just par-for-the-course campaign rhetoric. It went far beyond that.

But were Biden to become the Democratic nominee, this is the kind of primary attack that could be difficult to shake. How do Democrats argue that a man who would be 78 years old upon his inauguration in 2021 and who they said lacked acuity during the campaign would be a steady leader for the country? It’s easier to dismiss “crazy” and “dangerous” as overheated campaign rhetoric. What about the argument that Biden just isn’t sharp? You’re then going to tell people to trust him to engage in high-stakes diplomacy with foreign leaders and make life-or-death decisions?

Democrats will dismiss this by pointing to the man in the White House. They have long suggested there are signs of deterioration with Trump, who is also in his 70s. Trump’s speech certainly meanders, and he either doesn’t process basic facts or doesn’t care about misstating them. They’ll argue it’s unfair to focus on Biden’s flubs when Trump has so many of his own.

But the more they suggest Biden may not be a steady leader capable of “carrying the ball across the line” in the campaign, the more difficult it will be to draw that contrast with Trump. Democratic Party leaders have to be scared about the direction in which this is going.