It wasn't a "gaffe."
Senator Kerry has to distance himself from his radical pro-abortion stance because abortion has declined in popularity over the years.
New voters have seen 3D sonograms that clearly show that the child is alive as early as 7 weeks.
Older voters, such as Baby Boomers, are likewise less in favor of abortion now that they are becoming first time grandparents.
That convergence has put quite a squeeze onto the pro-abortion zealots, and it has certainly made it more difficult for a liberal to carry the 38 states that GWB won in 2000 (which are all that he needs to re-win his election this year, too).
Senator Kerry has to distance himself from his radical pro-abortion stance...I think Kerry's statement yesterday, contrary to how it looks on the surface, actually expresses the most radical stance on abortion a person can take. In effect, he said "yeah, fetuses are people too but let's kill them anyway". This is the same position held by the most radical pro-death feminists who see abortion as a kind of empowering she-goddess blood rite, knowing full well that it takes the life of a human being. It's a postition closer to pagan hihilism than pragmatic centrism. Most of the pro-abort crowd at least attempt to cover themselves with a figleaf of morality and reason by holding to the notion that a fetus is just a lump of tissue (and may therefore be justly disposed of).