Tom Holbrook does the math:

...the norm is for very little swing in candidate support following debates. Across all thirteen presidential debates the average absolute change in candidate support was 1 percentage point. There are a few notable exceptions, of course. Two that stand out are the second debate in 1992, following which George H.W. Bush lost 2 points, and first debate of 2004, after which George W. Bush lost 2.26 points. Other debates with above average ( but still small) vote shifts are the first debate in 1996 and the second debates in 1988 and 2000.

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