Debates rarely have an effect on the outcome of a presidential election.
A 2008 Gallup study found that between 1960 and 2004, there were only two years where debates made a difference in actual votes, notes Miranda Green. Instead, the most common outcome of the presidential debates is a slight popularity bump… Data from the Gallup study also saw no direct correlation between the winner of each debate and the winner of the presidency. The 2004 Kerry vs. Bush debate was cited as an example. Kerry was considered the victor of all three showdowns, but still lost the election.