IMO, Kerry-Edwards slime think that some hardcore bigoted militia type conservatives or religious zealots are going to not vote based on finding out Cheney's daughter is a lesbian.
The media exposure alone in this has got to throw it into the plus column for Bush as it is one of the most crass and distasteful campaign tactics played in modern politics...that and the Special Olympics flyers.
And, interestingly enough, one of the guys the left love's to hate, Jerry Falwell, is going all out for Bush, and this in spite of any disagreement he may have over some of the social issues. He made that very plain in a recent interview on Fox News. So, if the left thinks of Falwell is the epitome of right-wing Christian fanatical bigots and that was their reason for going after Mary Cheney, it ain't working.