Trump and his allies had a plan for how to hit Harris. Then he opened his mouth.

“They don’t have a narrative that they’re comfortable with about how to take down Harris,” said Chuck Coughlin, an Arizona-based political strategist. “He’s grasping around. I think he’s desperately grasping around with his instincts. I don’t think his team has any way to put their handle on this, and so he’s...