anchor: Once Congress agrees to regulate one sort of abortion because it is gruesome, the pro-lifers will immediately turn to another form of abortion and insist that it, too, be regulated, because it, too, is gruesome.
positive: The pro-lifers will immediately turn to another form of abortion, once congress agrees to regulate one sort of abortion for the reason of it being gruesome.
negative: Once Congress agrees to regulate one sort of abortion, it will shut the pro-lifers up for good.