Let us conclude Dred Scott had an overt appeal, as you mentioned, and a more profound though and veiled meaning, as Liz and others have seen. That's how I took it, and I rather suspect that's the way Bush intended it.
BTW, the fact he made this statement in St. Louis - where Taney handed down that same Dred Scott decision some 146 years ago - added something to the poignancy.
Also highlights a troubled culture corrupted by self-absorbed liberals in which defense of the unborn is discouraged.
According to the Liberal Credo, pro-life statements are considered "religious" and thus fall under the atrocity of political correctness.