Democrats worry that Supreme Court nominee Samuel Alito will vote against a constitutional right to abortion and against an expansive view of congressional power. But if the first prediction is accurate, they should hope the second one is too.

 Assuming Alito is inclined to overturn Roe v. Wade, the 1973 decision in which the Supreme Court started dictating the contours of state abortion laws, confirming him would leave a five-justice majority in support of the decision. Then Roe v. Wade's demise might be just one more vacancy away.

 That prospect should give Democrats a new appreciation for federalism.  They stand to benefit if Roe v. Wade's reversal returns the abortion issue to state legislatures.

 Surveys indicate that most Americans want abortion to remain legally available, although they disagree about exactly how loose the rules should be. The threat of new restrictions would energize Democrats and handicap Republicans, who currently can appeal to conservatives by proclaiming themselves pro-life without losing much support among moderates. When abortion is once again a live issue in every state legislature, they won't have that luxury anymore.

 Since most voters do not want the government to ban or severely restrict abortion, most state legislatures would not choose to do so in the wake of Roe v. Wade's reversal, although some might. Regulations would vary from state to state, and abortion policy would become another factor in the competition between states for residents and tax dollars. Instead of conforming to a single, nationwide policy, people could vote with their feet for the approach they liked best.

 But what if Congress tried to shut down the laboratories of democracy, something it has been known to do in areas ranging from speed limits to education? If Congress decided to restrict or ban abortion at the federal level, it would be the Supreme Court's job to enforce the constitutional distinction between local and national matters.

 This scenario is by no means purely hypothetical. The Partial Birth Abortion Ban Act of 2003, purportedly grounded in Congress' power to "regulate Commerce with foreign Nations, and among the several States," prohibits the abortion procedure known in the trade as "intact dilation and extraction." The law applies to "partial birth" abortions "in or affecting interstate or foreign commerce," which is boilerplate meant to encompass all uses of the forbidden method.

 In July the U.S. Court of Appeal for the 8th Circuit ruled that the ban is inconsistent with Roe v. Wade because it does not include an exception for abortions deemed necessary to protect the mother's health. But even in the absence of Roe v. Wade, the law would be unconstitutional because Congress has no authority to regulate abortion, a power the Constitution reserves to the states.

 This is not the sort of distinction Democrats, traditionally fans of a powerful federal government, usually worry about. But perhaps they should start. After all, Republicans have been running Congress for a decade and the executive branch for half that long, and they are not shy about using Leviathan for their own ends. A minority party should recognize the advantages of federalism, which lets New York and Utah go their separate ways on contentious matters such as abortion instead of subjecting them both to decisions made in D.C.

 In 2003, after the U.S. Court of Appeals for the 9th Circuit sided with two patients who argued that the Constitution allows California to legalize medical marijuana without interference from Washington, their lawyer, Boston University law professor Randy Barnett, declared that "federalism is not just for political conservatives." The same observation could be made regarding federal attempts to override Oregon's law permitting physician-assisted suicide, an issue the Supreme Court is now considering.

 In both cases, left-liberals found themselves standing up for federalism while conservatives defended an overbearing central government. If the Republicans retain their grip on the levers of national power, perhaps this will be the shape of things to come.