I am thinking one plausible reason that candidates this election season tried for a long time to play clean campaigns was to avoid as much as possible any criticism and bigotry associated with each candidates respective race, gender, religous or ethnic background.
On the Democratic side you have Hillary and Barack. A strongly racist or bigoted person would most likely have a difficulty voting for a woman or African American. John Edwards although probably not viable at this point for president (V.P perhaps?) made it clear on many occasions in 2007 that if a certain voter would vote for him based solely on the fact that they would not vote for a woman or an african american that he did not want their vote. And so amongst the top three democratic candidates, there is no clear candidate for people with strong bigoted or racist views.
The republican side have a bit more to offer towards people with bigoted or racist views but not much. It's interesting with Romney how on one side people may be prejudiced against his religion because they don't see it as Christian, while others have claimed the religion is racist based on their own interpretration of church policy prior to 1978. You also have Rudy Guiliani who was particularly welcoming of any person (legal or illegal) who would work in New York City, many people with bigoted view may take serious issue with this. McCain and Huckabee also rather soft on illegal immigration and support what many in the anti-illegal immigration movement see as a rather easy path to citizenship.
McCain and Huckabee have sharpened up there stances and given greater emphasize to ''securing the border'', from a certain angle this could be an indirect way to offer appease bigoted voters as illegal immigration has become an issue at the forefront in surprising places like New Hamphsire and South Carolina where immigrant populations from Mexico are small but growing rapidly.
Guiliani comes of from a Italian Roman Catholic and McCain from Scotch-Irish descent (according to his website). In many parts of the country it may surprise people the strong prejudices and animosities which existed in the past towards people of these respective groups. Things are different now than they were then, which may be a sort of omen for what the election could like in another 4,8,16 or 32 years?
Either way with peoples views as they are today there may be no real clear candidate for the bigoted vote to flock to.