Marco Rubio couldn’t execute a winning campaign against Donald Trump in the 2016 Republican primaries, but he did identify the way to get under his skin. By talking about the diminutive size of Trump’s hands, he actually got the future president, during a globally-televised debate, to assure the world that he doesn’t have a small penis.

It wasn’t shocking that Trump did this; it was shocking that he won the presidency despite doing it.

This history shows that making Trump act like a fool is not any guarantee of beating him. But perhaps the real problem is that Rubio’s stunt was a one-off event. Trump’s opponents were almost always on the receiving side of mockery rather than the ones dishing it out.

Joe Biden isn’t making this mistake. There are some legitimate concerns about whether Biden is slipping a little mentally as nears his eightieth birthday, and this was supposed to be central to Trump’s plan of attack. But Biden and his allies are destroying the president almost every single day by attacking his physical and mental health, as well as his moral courage. And this seems perfectly suited to making Trump defensive and ridiculous.

He hates nothing more than people thinking he’s not physically strong, virile, and courageous, which is why can never seem to let these attacks go. When I was a taking political science classes in college, I learned that Richard Nixon made a big mistake when he said he was “not a crook” because he amplified the charge that he was a crook. That’s what Trump did when he spent 14 minutes on stage in Tulsa explaining why he can’t drink water with one hand or walk normally down a mild incline. Sure, a lot of people had seen some viral videos mocking his appearance at West Point, but he jacked up how many people knew about it by several million.

He’s getting the worse end of the fitness argument, which has the doubly brutal effect of neutralizing his attacks on Biden. He winds up sounding like the kid who says “I know I am, but what are you?”

Ordinarily, I might have some qualms about ageist attacks on Trump, but he’s been making attacks on anyone who is weak or disempowered since he began his first campaign in 2016: accusing immigrants of carrying disease, saying Jeb Bush has no energy, mocking disabled reporters, suggesting that Hillary Clinton is at death’s door, and now claiming that Biden is senile.

This is a guy who got out of Vietnam because he had sore feet. He’s completely out of his tree half the time, and now it looks like he’s got some significant physiological problems. He lives in a glass house on the issue of physical and mental fitness, and I suggest that people throw every rock they can find at him.