The U.S. Supreme Court will hear arguments in two cases today on the legality of the federal ban on “partial-birth” abortions.

The court is asked to decide whether a 2003 law outlawing the procedure is unconstitutional because it lacks an exception for the pregnant woman’s health. This is not a new question for the court, which six years ago struck down, 5–4, a Nebraska law that also lacked a health exception.

But the Supreme Court’s roster has changed dramatically since the court decided Stenberg v. Carhart, the Nebraska case. Justice O’Connor, who sided with the majority in the case, retired this year.

Her replacement, Justice Alito, could cast the deciding vote to uphold the Partial-Birth Abortion Ban Act. Alternatively, Justice Kennedy may switch from his position as a dissenter in Stenberg to vote against the law if he finds the question worth deciding to be how much deference the Supreme Court owes Congress.

In some ways, it is surprising that legal observers could be most interested in Justice Kennedy’s response to today’s arguments, which are scheduled to last two hours.

Justice Kennedy wrote a mournful dissent in Stenberg, chiding the majority for considering the “partial birth” abortion procedure from the vantage of the “abortionist” instead of from “the perspective of a society shocked when confronted with a new method of ending human life.”

The “partial birth” abortion debate returns to the Supreme Court this time laden with constitutional questions that were absent from Stenberg. One of the questions is how much deference the court must give to Congress’s finding that a health exception is unnecessary because the procedure is never required to protect a pregnant woman.

In Stenberg, the Supreme Court did not arrive at that conclusion. The court could question whether Congress chose to substitute its own constitutional judgment when it should have deferred to the court’s findings in Stenberg.

In cases in other areas, Justice Kennedy has carefully protected the court’s role as the arbiter of the Constitution.

In an effort to cast doubt on the integrity of Congress’s findings, 52 members of Congress who oppose the federal ban submitted an amicus brief saying that Congress’s investigation of medical data was results-driven.

“The decision to omit a health exception was not based on any intervening change in medical fact, but founded upon a desire to challenge the prevailing constitutional standard that Stenberg affirmed,” the brief reads.

Whether “partial birth” abortions are ever called for to preserve the health of the pregnant woman is debated by both sides. So is the precise definition of “partial birth” abortions and the frequency with which they are performed.

Whether the court is inclined to take offense at Congress’s efforts to write over Stenberg is uncertain. There is nothing necessarily illegitimate with Congress asking the court to take a second look at a issue, one observer, law professor Eugene Volokh at the University of California at Los Angeles, said.

Also, the Supreme Court may be inclined to give more deference to a law of Congress than the laws of state legislatures, Mr. Volokh said.

The deciding factor, legal observers say, could be more mundane: a court takes its precedent seriously and is hesitant to overturn itself.

With that in mind, attorneys for Planned Parenthood Federation of America submitted a brief to the court, urging the justices to stand by Stenberg “regardless of whether a majority of this court agreed” with that decision.

The two cases are Gonzales v. Planned Parenthood and Gonzales v. Carhart. Both federal appeals courts to have heard one of the cases ruled the federal ban was unconstitutional.