Liberals who see the retirement of Sandra Day O'Connor as a giant step toward the overturning of Roe v. Wade are taking solace in two facts: For now, the U.S. Supreme Court seems to have five votes to uphold Roe; and even that precedent's demise would leave abortion legal in states that do not outlaw it themselves.

But, as Justice Harry Blackmun wrote 16 years ago, "a chill wind blows." The numerical majority may not be as safe as many hope. And more worryingly, recent decisions on federalism -- decisions applauded by liberals -- may well pave the way for a post-Roe national ban on abortion.

Let's begin with the arithmetic. The current Court is divided 6-3 on abortion. The supporters of Roe -- or, at least, supporters of what the majority in the 1992 decision in Planned Parenthood v. Casey called Roe's “core holding” -- are O'Connor, John Paul Stevens, Anthony Kennedy, David Souter, Ruth Bader Ginsburg, and Stephen Breyer. Even if O'Connor's successor votes to overturn Roe, the case will remain the law of the land.

But just barely and, perhaps, not for long. Kennedy broke with the other pro-Roe justices to dissent from the Court's 5-4 decision in the 2000 decision invalidating Nebraska's “partial-birth” abortion ban. A new anti-Roe Justice would shift the balance on that issue at least.

Moreover, one cannot be confident that the remaining five pro-Roe justices will all stay on the Court for the entirety of Bush's second term or that a pro-choice Democrat will win the White House in 2008. More broadly, although liberals and conservatives alike have termed the appointment of O'Connor's successor pivotal because of her frequent role as the swing vote on the Rehnquist Court, in an important sense, every appointment is pivotal; whether a given justice is on the left, center, or right of the Court depends entirely on where all the other justices are.

Accordingly, liberals who care about the direction of the Court must be prepared to fight conservatives on every appointment. But given Republicans' margin in the Senate, their party leadership's willingness to eliminate the judicial filibuster, and the uncertainty over whether the compromise agreement among centrist senators will hold, liberals should also begin making contingent plans for a post-Roe future.

At first blush, that future does not appear utterly bleak. In the immediate aftermath of a decision overturning Roe, the legality of abortion would be up to the states. We could expect that, in general, abortion would remain legal in clearly blue states like California and New York but would be prohibited in clearly red states like Texas and Louisiana.

But even this scenario may be unduly optimistic. Today's Republican Party pays at most lip service to the notion of limited national power. The Justice Department was all too eager to interpret federal statutes to override Oregon's law permitting physician aid in dying and California's medical marijuana law. Likewise, Congress (including many Democrats) brushed aside the obvious federalism objections to its extraordinary intervention in the Terri Schiavo case.

Thus it is not alarmist to predict that within weeks -- if not days or hours -- of a Supreme Court decision overruling Roe v. Wade, Congress would enact legislation outlawing most abortions nationwide. At that point, the fate of legal abortion would depend on the justices' views about the limits of congressional power.

Over the course of the last decade, the justices of the Rehnquist Court have divided over the scope of congressional power, with the more liberal justices taking a broad view and the more conservative justices generally taking a narrower view. But as last month's medical marijuana decision, Raich v. Gonzales, illustrated, this description is too pat.

In Raich, the Court's four most liberal justices voted to sustain the application of the federal drug laws to the intrastate cultivation and personal medical use of marijuana. Expressing the traditional deference that liberals show to Congress on such matters, Stevens wrote that because there is an interstate market for marijuana, Congress may legitimately decide what exceptions, if any, to make to its general prohibition.

The liberals were joined in Raich by Scalia and Kennedy, who had previously sided with Rehnquist, O'Connor, and Thomas in reading the scope of congressional power to regulate interstate commerce more narrowly. Although many liberals were dismayed by the bottom line in the medical marijuana case, liberal Court-watchers were relieved, for we understood that the states' rights counter-revolution that Rehnquist had started would no longer pose much of a threat to the Endangered Species Act and other environmental legislation.

That sense of relief will be short-lived if the Court overturns Roe and then relies on the medical marijuana precedent to uphold congressional power to ban abortion nationwide, even in the states that permit it. In a post-Roe future, we may face a painful choice between abortion being illegal throughout the country and the invalidation of many federal laws protecting the environment.

To be sure, that future does not face us immediately. Abortion will remain legal throughout the land at least until another pro-Roe justice leaves the Court. And even in the medium-term post-Roe future, a majority of pro-Roe and states' rights justices might be cobbled together to invalidate an act of Congress banning abortion nationwide.

But the long term is not all that far away. The average age of the justices remaining on the Court after O'Connor's retirement is over 70. Democratic senators and liberal interest groups who must decide how to use the limited weapons at their disposal to combat extremely conservative nominees need to begin thinking about the dangerous intersection of federalism and rights cases today -- before it is too late to make a difference.

Michael C. Dorf is the Michael I. Sovern professor of law at Columbia University in New York City. His 2004 book, Constitutional Law Stories, is published by Foundation Press and tells the stories behind 15 leading constitutional cases. His next book, No Litmus Test: Law and Politics in the Twenty-First Century, will be published by Rowman & Littlefield in early 2006.