Yesterday, John Sides wrote about some interesting studies exploring the effect the media have on voter perceptions of presidential debates. One experiment showed that when you expose people to post-debate commentary, it significantly alters their perception of who won the debate. This is something researchers have known about at least since 1976, when the public at first didn't see Gerald Ford's "gaffe" about Eastern Europe not being under Soviet domination as any big deal (apparently, they realized he was speaking more aspirationally than anything else). Immediately after the debate polls showed the public evenly split on who had won, but after a few days of coverage of the "gaffe," the polls shifted dramatically, with many more people saying Carter had won.
As I've pointed out many times, what persists in our memory about presidential debates are only those moments reporters choose to keep reminding us about (I wrote about it in this book—still relevant eight years later!). But there's an important question to keep in mind when you consider the question of the media's influence: Does it matter?
If the question you're asking is, "Which candidate do voters perceive as having won the debate?" then yes, the media matter a great deal. But that's not a particularly important question. Similarly, a debate can "matter" without it actually changing the outcome of the race. It can inform people, or bring up a new issue we haven't much thought about in a while, or show us a side of one or both of the candidates we haven't seen before, and that can be good even if no one's vote gets changed. On the other hand, a debate full of stupid questions ("Mr. Romney, you're trailing in the polls. Why have you failed to connect with the voters?") and evasive answers can make people more cynical about politics, which matters too. And the media's influence can matter if they ignore everything that was interesting, edifying, and revealing about a debate and instead spend a week talking about whatever clever line one candidate or another tossed off. Which, if the past is any guide, is what we're most likely to get.