The ACLU is more committed to “abortion rights” than it is, these days, to anything that is actually in the Constitution, so it’s no surprise to see its deputy legal director in the New York Times urging the Supreme Court not to “undermine” that right.
Louise Melling’s argument is that allowing legislatures to protect unborn children — she doesn’t put it that way, of course — would compromise the Court’s legitimacy and integrity. These goods require consensus decisions rather than 5-4 splits, and incremental rather than radical change, and respect for precedent. I will admit to having doubts about how deep the ACLU’s commitment to any of these things truly is. Note also that the Court’s legitimacy does not, on this telling, have anything to do with sticking to the words of the Constitution.
Conservatives and libertarians disagree with one another about all kinds of constitutional questions, but liberals seem much more likely than people on the Right to engage in arguments about what the Supreme Court should do in constitutional cases that don’t even bother to mention any provision of the Constitution. I’m sure that in many cases the authors don’t even notice the omission.