The abortion issue is a federal issue. It derives this status from the Declaration of Independence where life is listed as being the first of the unalienable rights. If abortion was merely a states issue, it would be possible for it to be legal in one or more of the states. Since the right to life is an unalienable right, it is not possible for state legislators or voters to confer these rights, or to deny them.
The Declaration and the Constitution are taken together as being our founding documents. Upon them, all else rests.
I've heard him speak to this on numerous occasions. He says the Declaration of Independence answers the "why" question and the Consitution answers the "how" to accomplish what was laid out in the Declaration. Without BOTH, the meaning gets lost.