Good grief. Murder is a moral term; it means a killing that is unjustifiable. Where do you get the idea that it's a "legal construct"--only?
See, you are falling into that trap again. If that is all the definition means, then Michael Moore is correct when he labels the killing of innocent Iraqis as "murder" because he sees it as unjustifiable as well. But he's not correct and people who apply the label "murder" to abortion are not correct either. Here's the American Heritage definition: "The unlawful killing of one human by another, especially with premeditated malice." Notice the "unlawful" bit. What law does an abortion break? And yes, it is a legal construct. If you call someone a murderer and they have not been convicted of such or you do not use the "alleged" caveat, then you can be sued for libel or slander, whichever is appropriate. It is a specific legal term.
Use it if you want though, but don't base your argument on it. You can't say "abortion is murder therefore it is evil." A better path would be to explain why it is evil since it is unfortunately very much legal in this country -- something both Bush and Rice would like to see changed.