I do feel that there were voters who changed their votes based on the Seamus the Dog incident as in our society pets, especially dogs, are viewed as family members by many people. Putting a baby's car seat on the roof for the same reasons would be seen as very wrong. Not only that but the subject of animal abuse is very prevalent these days and putting your dog carrier on the roof of your car, where it doesn't have any protection from the wind can be seen as abuse.
Some people are very adamant about animal cruelty and I believe this incident clearly displays that Romney did in fact abuse an animal. Since people hold these values so strongly I believe it is possible that people changed their votes upon hearing about this incident. However, this isn't the only thing wrong with Romney, so he can't blame everything on this one incident.
I do not think that people changed their votes based on the Seamus the dog incident in 2008. I think that a lot of people were going to vote for Romney or not, and the Seamus the dog incident didn't do anything to change their decisions. I think it was an incident used for political reasons.
It is highly unlikely that the "Seamus the dog" incident caused any voters to change their minds during the 2008 elections. The "Seamus" story, a tempest-in-a-teapot if ever there were one, may have confirmed the negative image of Mitt Romney that many had already adopted, but in and of itself - especially given the weight of the issues at hand - the story was of insufficient substance to influence voter's decisions.
Despite all the supposed happenings and events that were thought to move the polls during the election, the actual demographics did not vary much during the campaign at all. Everyone pretty much knew from the start who they were likely to vote for as is, and I don't actually think many heard of the dog incident anyway, much less let it sway their vote