The plan is to remind voters that this will be a close election and every vote matters.
However, the debate is irrelevant. On the one hand, you have a liar who hates immigrants and wants to benefit wealthy Christians. On the other hand, you have a lousy debater who respects the rule of law and protects women’s rights.
Nobody faced with this choice is going to ask, “Hmm, just how lousy at debating are we talking?”
Biden’s polls have been fairly static or even slightly improved over time.
The other Democrats have done nothing to win over new voters, so there is no reason to think they would poll better against Trump today.