Then during the health-care segment, Mr. Biden said, “The plan we’re hearing discussed is the Biden plan.” Then he said, “The plan is going to cost at least $30 trillion over 10 years,” with no indication that now he was talking about Elizabeth Warren’s plan, not his.

Here’s a single Biden sentence from the debate: “I would eliminate the capital gains tax—I would raise the capital gains tax to the highest rate, of 39.5%.” And this: “My son made a judgment. I’m proud of the judgment he made.”

Mr. Biden’s supporters say he did fine, but fine isn’t going to be good enough. With apologies, a sports metaphor is apt. Joe Biden looks like a pro in training camp—running at half speed, joshing with teammates, showing brief flashes of former skills. Democrats who think Mr. Biden will get better than this training-camp competence are deluding themselves. Come next year’s October presidential debates, Donald Trump would flatten him.