Matthew Yglesias is correct to argue that the influence of campaign gaffes is probably overstated:
One thing I wonder about is how much do "campaign gaffes" really matter? My guess is that their perceived importance is mostly an illusion. I mean, people point to plenty of examples of campaigns that lost, in large part, "because of" this or that gaffe or damaging random thing dredged out of the record but you never see an example of a campaign that won because it successfully avoided gaffes.
As I've argued, the fundamentals (the state of the economy and war) drive presidential election results, but the process by which this occurs is not entirely clear. Who fills the void? Reporters and pundits who (a) see their job as interpreting the political process for citizens and (b) have a strong economic incentive to create an entertaining drama that will attract an audience. These journalists collectively end up constructing a plausible narrative to "explain" the trajectory of the campaign and its eventual outcome of the campaign. This narrative is often built around media-generated controversies that have little influence on the outcome.
Campaigns also tend to be seen as a referendum on the skill and quality of political candidates, which are also closely linked to gaffes. In almost every case, losers are seen as bad candidates and winners as good candidates even when we would have expected the actual outcome in advance. For instance, the collapse of Michael Dukakis in 1988 may have been because he was a bad candidate who made mistakes like riding in a tank wearing a silly helmet, but it also coincided with George H.W. Bush converging to his expected level of performance given the fundamentals.
With all that said, however, it's worth distinguishing between epiphenomenal gaffes and those with deeper political and cultural significance, especially when the latter is reinforced in paid media. While largely incoherent, Obama's statement about downscale whites in small town Pennsylvania was offensive and condescending in its phrasing. If enough money is spent drilling that message into the heads of lower income white voters, Obama's performance in the general election could be significantly affected.