Last week South Dakota Governor Mike Rounds signed into law a bill banning most abortions, setting the stage for a full-frontal assault on the Roe v. Wade decision that so deeply poisoned the politics of the late 20th century.

Don’t expect Roe v. Wade to get overturned anytime soon. These things take years, and at last count five of the nine justices on the Supreme Court are considered pro-choice. Even if President Bush were to succeed in appointing one more justice to the Supreme Court, giving conservatives an outright majority, they might be reluctant to throw Roe out root and branch.

And not just because the judicial conservatives profess deep respect for precedent. They also understand something about politics. Rejecting Roe by a narrow margin would be seen by a sizable chunk of the population as a judicial coup, provoking a reaction that could put a liberal in the White House and cut short the emerging conservative dominance of the court system.

Roe itself was decided by a 7-2 majority in 1973, with two moderately conservative Republican appointees, Chief Justice Warren Burger and Lewis Powell, voting with the majority. In the landmark Brown v. Board of Education decision in 1954, which spelled the death of legal segregation, the court deliberately stalled until Chief Justice Earl Warren felt assured of a unanimous decision. Even then the decision was met in the South with “massive resistance.”

Even if the South Dakota law is upheld, the effect would not be to make abortion illegal but to return the matter to the states. Many were moving towards liberalization of abortion well before Roe v. Wade preempted the question. And while the South Dakota law sounds tough, explicitly allowing abortions only to save the life of the mother, the actual wording reveals a potentially gigantic loophole. As the governor interpreted it,

“[The bill] does not prohibit the taking of contraceptive drugs before a pregnancy is determined, such as in the case of rape or incest.”

That would seem to permit “Plan B” morning-after pills. While the intention may only have been to allow prevention of pregnancy from rape or incest, the public may see only a thin line between such pills, which prevent implantation of a fertilized egg, and outright abortion pills, like RU-486.

There is talk in South Dakota of a referendum to repeal the new abortion law before it gets to the courts. But Mississippi and several other states are waiting in the wings with anti-abortion legislation of their own. The sense of frustration among prolifers may be so strong that there is no stopping an all-out court challenge.

But the price for this impatience could be high. The political energy from which conservatives benefited post-Roe will quickly shift to the liberal side. Already there are signs of split within the anti-abortion movement itself. A petition drive to ban abortion in Michigan is being pushed by a hard-line splinter group, Michigan Citizens for Life, but not by the “mainstream” Michigan Right to Life.

Impatience also could jeopardize the solid progress – abortion declined nationally by 17% in the 1990s, and by up to half in states like Michigan between 1987 and 2003, it has been estimated – that anti-abortion forces have made by gradually de-legitimizing abortion while chipping away at Roe. This might be a good time for conservatives to choose prudence over Utopianism.

Mr. Bray is a Detroit News columnist.