The second is called a "logical slippery slope." When one thing is immoral, and a second is logically similar in a morally relevant way, the moral quality of the one slips over into the other. Murder is immoral, and some think capital punishment is similar enough to murder to make capital punishment immoral too. A logical slippery slope can slip in either direction. If you have a morally acceptable thing, and something else is logically similar to it, then arguably the second thing becomes morally acceptable, too.
The validity of a logical slippery slope depends on the similarity of the two things being compared. This is why I think the word "abortion" is misleading in "partial-birth abortion." The differences between this procedure and infanticide are only a few inches of physical location and a few seconds from officially recognized birth. What is the moral distinction between a born baby and a baby with only its head left in the birth canal because the birth has been stopped?
The Real Reason
The partial-birth abortion question places America at a critical juncture in the sanctity of life debate. Which way will we slide?
When you start with the view that abortion is a sacred right, then by reason of a logical slippery slope, you must argue for anything called abortion. "All abortions ought to be legal. D&X is an abortion. Therefore, D&X ought to be legal, too." Slide a bit further and it looks like this: "If partial-birth abortion is moral, and partial-birth abortion is essentially the same as infanticide, then infanticide must be moral, too."
We could, however, regain our moral sanity and slide the other way. If this atrocity is made illegal, it doesn't take a rocket scientist to figure out that the very same thing is happening out of sight inside the womb in every late-term abortion. If the first is morally grotesque and illegal, then the second is grotesque and should be illegal, too.
Pro-abortionists know this, and that's why they're fighting so hard against the federal ban. They want freedom of choice at any cost, regardless of the moral consequences. When they raise the alarm, "This is the first step towards making all abortions illegal!" we finally have their real objection to the federal ban.
It's a legitimate fear. Once one realizes D&X abortions are clearly immoral, it's going to be hard to justify any late-term abortion. On the other hand, if it's clear that a living human child is being butchered, yet D&X remains legal simply because abortion is legal, then even the hardiest libertarians should realize there's something terribly wrong with the current state of abortion law.
No Clear Moral Intuitions
The question we now face is, "Which way will we slip?"
Will the Supreme Court, will Rudy Giuliani admit that there are no good reasons for partial-birth abortion? Will we admit D&X is really infanticide by another name? Will we then recognize that any late-term abortion under cover of the mother's womb is enough like D&X to be immoral, too? If partial-birth abortion is de facto infanticide, what meaningful moral distinction is there between infanticide and any other late-term abortion? They all destroy a little human being.
Or will we slide the other way? Will the public, will the courts admit that pro-lifers are right that partial-birth abortions are virtual infanticide, but then conclude that since abortion is moral--and here we slip the other way--then infanticide must be moral, too? Some have already proposed this.
We can go either way. That's why we're at such a critical juncture.