Roe v. Wade: How Trump's Appointment of a Supreme Court Justice Might Affect Abortion Rights

President Trump plans to name his pick for the Supreme Court tonight..

— -- President Donald Trump plans to announce this evening his pick to fill the vacancy left on the Supreme Court by the death of Justice Antonin Scalia in February of last year.

Trump, whose position on abortion rights has shifted over time, told CBS News' "60 Minutes" in November that he is "pro-life" and would appoint individuals who hold the same position to the nation's highest court. He said of the landmark abortion rights decision, Roe v. Wade, "If it ever were overturned, it would go back to the states."

While the replacement of Scalia with another conservative justice is unlikely to realign the Supreme Court in such a way that Roe v. Wade could be threatened, the replacement of additional justices could have major consequences, according to Kate Shaw, an ABC News contributor and a Cardozo School of Law professor. We asked Shaw to help explain how this might work, and she responded via email.

What is Roe v. Wade, and what did it do?

Kate Shaw: In Roe v. Wade, the Supreme Court held that the Constitution protects a woman's right to decide whether to terminate a pregnancy. At the time Roe was decided, 1973, many states criminalized abortion, and Roe struck all of those criminal statutes down. Following the decision, states weren't able to prohibit abortion outright, but they do have the power to regulate it.

What can states do to regulate abortion?

KS: Roe held that while the Constitution protects a woman's right to decide for herself whether to continue with a pregnancy, states also have a strong interest in regulating abortion, both to protect women's health and to promote potential life.

Let's assume Trump appoints a person ideologically in line with Scalia. What will be the court's ideological makeup at that point?

KS: Justice Scalia was a strong opponent of both the Roe and Casey decisions. A jurist who shares Scalia's views in this area would maintain the status quo, which is four strong votes in favor of Roe, four justices who don't believe the Constitution protects the right to abortion, and Justice Anthony Kennedy, who has voted with either blocs when it comes to abortion (though it's significant that his most recent vote in an abortion case was with the court's liberals). So statutes like the Texas clinic regulation, which Kennedy joined the liberals in striking down, would remain invalid.

It could also happen more gradually, and I think that's more likely, as the court tends to prefer to act incrementally. The court could uphold a series of abortion regulations, gradually weakening the foundations of the Roe and Casey rulings and then finally overruling those cases.

But — and this is a big "but" — there is no absolute guarantee that Roe would be overruled, even with three or more Trump appointees. It's at least possible that the court would decide to adhere to Roe on the grounds that, as Justice Louis Brandeis wrote in 1932, "In most matters it is more important that the applicable rule of law be settled than that it be settled right." Even a conservative court could decide it would simply be too disruptive to overrule this long-settled precedent on such a divisive social issue. That's essentially what happened in Casey, when everyone thought Roe would be overruled but it wasn't.

Would a ruling against Roe have immediate nationwide impact?

KS: A decision overruling Roe would allow states to ban abortion, but wouldn't require them to do so. And the big question would be which states would do that. A number of states have laws on the books that would essentially ban abortion immediately if Roe is overruled. Other states would likely move quickly to pass such laws. In all, over half of the states would likely eliminate or severely restrict access to abortion inside their borders if Roe is overruled.

In addition to what happens in the states, it's conceivable that Congress could attempt to ban abortion nationwide. Without any constitutional guarantee of a right to abortion, the main constraints on Congress would be political, not legal.

This story has been updated