Many recent studies and commentaries have maintained that past debates have not had a decisive impact on the election. A lot of memorable moments, one liners, post-debate analysis of winners-losers, but not much in the way of "game-changing" moments. Very little that altered the outcome of the race, according to many experts.
Certainly the John F. Kennedy-Richard Nixon debates were key to getting JFK the exposure he needed and the new TV format was a game changer. One could argue that the Jimmy Carter Carter-Ronald Reagan debate solidified Reagan's victory and helped grow his margin.
But many of the debates have simply reinforced viewer's opinions and changed few minds. Viewership, of course, is another issue. And, this year, there are three debates plus the vice presidential debate. So there are more opportunities for a mistake or miscalculation.
The reason these debates may matter is pretty simple: There is high interest in this election, voters are laser focused on the economy and their personal situations, and want to know who can best stand up for them. There is a bit of desperation—more than usual—and the candidates will have a chance to transcend the plethora of political ads and the heated rhetoric that have gone on for so many months.
In short, the audience could well be higher for this first debate—over 70 million—and may contain more leaning or undecided voters.
In addition, the role of social media in analyzing the debates, more commentary, greater "chatter" could add to the impact. I wouldn't predict any actions by the candidates that result in a "moment" that changes the trajectory of the race but I could see the debate influencing that small number of undecided voters.
Maybe that is why both candidates are spending so much time in rehearsal mode!