This is a fairly academic article, but fascinating. An academician, Mendelberg, analyzes how the use of the Willie Horton ad was so successful. Then went on to explain how it worked and how to fight against it.
Thrust and Parry
How Kerry can fend off this year's version of "wedge politics"
by Chris Barrett, Contributor
http://gadflyer.com/articles/?ArticleID=72snip..
It's worth pausing here to explain this unusual finding. What Mendelberg has found is that voters must be confident that their motives in supporting a candidate are unimpeachable. If they believe that a candidate has become associated with an unsavory political view, they will abandon that candidate, even – and this is important – if part of them agrees with that view.
From this finding, Mendelberg draws two prescriptions: (1) political candidates can turn an implicit campaign into an explicit one by calling it what is; and (2) the counter-claim must be broadcast and debated to encourage voters to re-evaluate the issue.
A similar challenge now emerges in the form of President Bush's gathering campaign against gay marriage. The issue of sexuality pushes a great many of the same implicit buttons for voters as does race, but the key ones are that those affected are a minority, and that the campaign has that most precious asset of all: deniability. We have already heard, and will hear again, that the issue is not one of discrimination, but the protection of the institution of marriage. In this way, a deeply negative campaign is "laundered" into a positive one.
snip...
Which brings us to John Kerry. Candidate Kerry has thus far adopted a compromise position: he is against gay marriage, but for civil unions. He opposes President Bush's constitutional ban on gay marriage. This is the conventional response to a wedge attack, staking out a nuanced policy position and probably hoping the issue will go away. But the problem with wedge issues is that the other side won't cooperate in making the issue go away. President Bush may have been pressured by the right wing of his party to raise the issue, but Kerry's discomfort no doubt assures him that in the end it will work to the Republicans' advantage.