Everybody’s entitled to his opinion, it is said, but not to his own facts. Congress, however, decided to make up its facts when it wrote a law in 2003 banning a procedure that opponents call “partial-birth abortion.”

Now the Supreme Court is considering the constitutionality of the law in a case that will turn in part on real facts — as found by judges — instead of made-up facts as found by members of Congress playing to political constituencies. And the case will provide a key test of whether the court’s newest members, Chief Justice John G. Roberts Jr. and Justice Samuel A. Alito Jr., will decide this case on the basis of the law and the facts — as they promised during their confirmation hearings — and not on the basis of their personal political views.

The debate over the sort of abortion at issue, now more than a decade old, has been misleading from the start. In medical terms, the procedure is actually called a “dilation and extraction,” D&X, or sometimes an “intact dilation and evacuation,” or intact D&E. The procedure calls for the woman’s cervix to be dilated, the fetus to be brought partly out of the vaginal canal and the skull to be punctured to allow for complete removal of the fetus.

Despite the term coined by anti-abortion groups, the procedure entails no “partial birth” — at least, not when performed in the second trimester of a pregnancy. At that point, a fetus is not viable: It cannot live outside the womb. When pressed, even anti-abortion groups concede that banning the procedure while allowing other abortion techniques would not increase the number of live births.

Without doubt, the procedure is controversial within the medical community. Doctors who use the technique say removing a fetus largely intact instead of dismembering it in the womb is safer for the woman because it involves less instrumentation inside her body. Other doctors say the procedure is not safe, in part because it may increase the incidence of preterm deliveries in later pregnancies.

Many doctors, however, simply recoil at the procedure for the same reasons that anti-abortion groups cite: It sounds gruesome. But when the Supreme Court struck down a Nebraska law banning the procedure six years ago, Justice John Paul Stevens aptly said that it was “irrational” to ban one procedure while allowing other, “equally gruesome” abortion techniques.

That 5-4 decision held the Nebraska law unconstitutional because it did not permit the use of the procedure to protect the woman’s health. Roe v. Wade, the Supreme Court’s landmark 1973 abortion rights decision, established that any regulation of abortion — even during a pregnancy’s final trimester — must allow such a health exception. Throughout more than three decades, that bedrock principle has remained unchanged.

Congress, however, still wanted to ban the procedure. Since lawmakers may not change a constitutional ruling by the Supreme Court, they did the next best thing: They tried to change the facts. The federal ban, passed in 2003, opens with 28 paragraphs of “factual findings” aimed directly at defying the court’s ruling.

Most broadly, Congress got around the need for a health exception by “finding” that the banned procedure was “never” medically necessary and was not “safe.” It found a “consensus” within the medical community against the procedure. It went so far as to say that the procedure is not taught in medical schools.

Each of those statements is simply wrong — as U.S. District Court Judge Richard Kopf of Lincoln, Neb., found at the start of an exhaustive, 474-page decision in September 2004 that found the law unconstitutional. Kopf said the record before Congress itself, along with the later evidence in the court case, showed the procedure to be necessary in some circumstances, safe and recognized by a “substantial body” of medical opinion.

Kopf’s ruling, upheld by the federal appeals court in St. Louis, was one of two cases the justices agreed to use to decide the constitutionality of the federal ban. Defending the law during arguments Nov. 8, Solicitor General Paul Clement acknowledged that some of the congressional findings were unsupported.

All eyes were on Justice Anthony M. Kennedy during the arguments. He had voted to uphold the Nebraska law, but in the new cases he closely questioned lawyers on both sides about the claimed need for the procedure. Many saw his questions as leaving an opening for him to change his vote from the previous case.

Roberts was also an active questioner, with some of his comments seemingly aimed at finding a basis for upholding the law. Alito said nothing and was seen frequently staring off into space.

Anti-abortion groups are counting Roberts and Alito as votes to uphold the law and hoping to hold Kennedy’s from the prior ruling. Applying Supreme Court precedent to the proven facts in this case, however, points inexorably to striking down the federal statute. Upholding it would mean that Congress could intrude on the court’s constitutional authority and — for Roberts and Alito — send a distressing signal that they did not leave their political agendas behind when they donned their justices’ robes.

Kenneth Jost is the Supreme Court editor for CQ Press.