It looks like an avenue that might work for democrats is to simply encourage Trump to talk more often. His rallies always start out well attended, but lately by the end of the speech, a significant number of people leave, baffled by his word salads. From a Guardian article:
Trump repeatedly broke away from his prepared speech about economics to make rambling claims that Harris was both a fascist and a communist, to attack her laugh as that of “a crazy person” and a “lunatic”, and to claim he was more beautiful than the vice-president. He also spent time debating aloud with himself how to pronounce the name of the CNN anchor Dana Bash.
By the time he stopped speaking 100 minutes later, a large number of the arena’s 8,000 seats had emptied. . . .
“He reminded me why I’m not going to vote for him this time,” said Jenny, a local businesswoman who did not want to give her full name because she didn’t want to alienate customers.
“I voted for him in 2016 and had a Trump flag in the front yard. I voted for him again in 2020 but didn’t put the flag out that time. I’ve been thinking of voting for him again because Biden’s been so bad for the economy and Kamala won’t be any better. But after listening to that, I’m actually afraid of Trump being president again. I don’t know what he was talking about half the time. Perhaps he was always like that but he seems worse, more unstable.”
As his mental state deteriorates, his rallies are going to hurt him more and more.