Americans saw Kerry as the baby butcher he is (embryo cloning for stem cells, abortion).
I don't think Mel affected anything. Other than it is nice to have a moral, family man in Hollywood.
Unfotunately, most Americans favor butchering babies for stem sells. If anything, this issue helped Kerry. His support for partial birth abortion, however, probably did hurt him, and his pro-abortion stand probably hurt him with Catholics.
The pro-life movement has done a decent job in moving the country to where many (if not most) Americans see it's wrong to kill a baby in the womb for the sake of convenience. We've got a long way to go in convincing Americans that it is unacceptable to kill babies created outside the womb for the sake of science.