It’s not difficult to imagine a scenario where either candidate uses the debate to take the focus off of his or her particular negative attributes and thus gains a boost in the polls. Gallup’s Frank Newport recently showed that for the past two months, most of the news Americans have heard about Clinton revolved around her emails or her health. Respondents also reported hearing the daily news about the Trump campaign – mostly where he traveled and his often controversial statements. So if either nominee uses the debate to effectively focus the race (and the media) away from these negative aspects (or onto the opponent’s negative attributes), that could cause a shift in the race.
That being said, the effects of a single debate are often temporary, and the polling after the debate season ends frequently looks similar to the polling before it began.
This is an updated and slightly modified version of a chart from Erikson and Wlezien’s book, “The Timeline of Presidential Elections.” Essentially it shows that polls (according to archives) from the week before the first debate closely track polling in the week following the final debate.