I agree with this article that the first debate will be critically important. While there are many factors shaping this presidential race, I am convinced that a key one is the perception that Biden is too old and has lost touch with reality. I honestly do not believe that Biden can win unless that perception is dealt with. This helps to explain his low approval rating and slippage in places, such as Virginia and Minnesota, where he should not slip to the point of making those states competitive.
If he shows that he is with it in the debate he can change the narrative a bit and making reverse it enough to give him an opening to catch Trump. If he looks like an old man who is losing his mental acuity' then the Democrats have to seriously consider replacing him. There are strong negatives in making that replacement but they have to seriously consider it if he does not do well in the first debate. Should be must-watch TV for those wanting to see if the election dynamic will change.