Biden's Debate Performance Was The Most-Discussed Event Of His Presidency Since Inauguration
As of now, it's his defining moment.
Debates, political historians tell us, are inconsequential. Allan Lichtman, a historian who predicts presidential election winners using a system of “keys” which has accurately forecast the outcome of 9 out of the last 10 elections, appeared on CNN recently to make this case:
“It’s a huge mistake. They're not doctors. They don't know whether Biden is physically capable of carrying out a second term or not. This is all foolhardy nonsense.”
Lichtman’s argument is, essentially, that incumbency is a much more important factor in determining the winner of a presidential election than debate performance, and there is no historical evidence that a debate performance moves votes significantly.
Based on public conversation about the President’s performance and his overall ability to handle the job for another term, I would be deeply concerned that this debate could prove to be the exception if I were President Biden’s team right now.
Take a look at the number of posts shared on each day of President Biden’s first term that mention Biden:

The debate generated more one-day conversation about President Biden than any other event, including his inauguration, and the numbers aren’t close:
There’s another warning sign here for President Biden: the #5 column here, the day that generated the fifth-largest conversation about the President of his entire term, was the day following the report from Special Counsel Robert Hur, in which Hur called President Biden an “elderly man with a poor memory.”
As of this writing, Republicans on the House Judiciary Committee just filed suit to force the release of the audio recordings of Hur’s interviews with President Biden (the transcript is already public, but audio is more effective than written text when creating attack ads).
What were people saying about Biden after the debate?
During the three-day period after the debate, 92.5% of posts mentioning President Biden from a randomized sample set of accounts located within the U.S. that did not have partisan signifiers in their bios referenced his debate performance.

Of those posts that mentioned Biden and referenced the debate, slightly more than half (50.3%) came from Trump supporters denigrating Biden, less than one-fourth (23.8%) defended Biden, and slightly more than one-fourth (25.9%) came from accounts that were not supportive of Trump but expressed concern about Biden’s fitness.

The biggest political crisis Biden has faced
Based on the sheer size of the public online conversation, and the composition of the conversation (in which only 23.8% of posts about the debate performance were defending the President), the debate performance appears to be a political crisis unlike any he has faced in the past. It’s far beyond the scope of this newsletter to recommend a course of action, but it does appear that without some drastic course correction or major event that upends the current narrative, the debate performance could become the defining moment of Biden’s presidency.