The prospect that the high court of Massachusetts will impose gay marriage on that state, has many traditionalists worried. They fear that unless they enact a constitutional amendment to ban gay marriage, America’s courts will impose them on the nation. Senate majority leader Bill Frist has already endorsed such a constitutional amendment.

Others have objected that there is no need for a national settlement of this issue. Dick Cheney said in the vice-presidential debate in 2000 that each state should decide for itself. Why should federal action be taken to hamstring the states, ask these latter-day federalists?

In theory, there is quite a lot to be said for letting states set the laws best attuned to their citizens’ particular needs and views. It is worth pointing out, however, that the people now urging a state-by-state solution have been happy to impose one-size-fits-all national solutions when they had the power. Opponents of the marriage amendment rejoiced when the federal Supreme Court overrode the state government of Texas, striking down its laws against sodomy, this spring.

Indeed, the courts have become a major enemy of federalism. The Supreme Court federalized abortion policy in 1973, to take one notorious example, and in 2000 it went so far as to nullify laws, enacted by most states, to ban partial-birth abortion. When imposing such national settlements, the Court frequently resorts to a rhetoric in which federalism is presented as a dangerously explosive force. It suggests that allowing state-by-state political debates on such charged issues as abortion policy would lead to civil strife, and even to violence. The Court presents itself as a unifying institution in American life. Of course, its seizure of power has not, in practice, calmed our political debates but rather inflamed them.

The courts’ sorry history on abortion may be about to repeat itself on gay marriage. In the real world, the likely alternative to a federal constitutional amendment banning gay marriage is not a rich democratic debate in which some states’ citizens decide to support gay marriage and some states’ citizens vote it down. It is that the federal and state judiciaries will impose gay marriage nationally without much regard for the sentiments of voters in any state.

The full faith and credit clause of the Constitution mandates that states defer to one another’s public acts and records. A marriage contracted in Arkansas is generally valid in Massachusetts. So what happens when a same-sex couple from Arkansas go to Massachusetts to get married? When they come back home, they will file a suit demanding that the government of Arkansas recognize their marriage.

If the government of Arkansas chooses not to do so, it will have only one defense. The Constitution explicitly gives Congress the power to make exceptions to the full faith and credit clause, and in 1996 Congress voted to let states refuse to recognize one another’s same-sex marriages. President Clinton signed the bill, called the Defense of Marriage Act.

But it may not be hard for activist litigators to overcome the Defense of Marriage Act. A federal court could strike the act down, finding that it was based on an unconstitutional “animus” against gay people—the same rationale that the Supreme Court used in its 1996 and 2003 gay-rights cases. Or an activist Arkansas state judge could find that while Arkansas has the notional right to refuse to recognize out-of-state gay marriages, its decision to exercise that right is illegitimate. The judge could find that the exercise of that right is also based on anti-gay animus, or that it is incompatible with some far-fetched reading of the state constitution. This would be an abuse of judicial power, of course. But it would be a smaller abuse than imposing gay marriage in the first place. If some judges are willing to do that, we have to assume that others will be willing to import their handiwork.

“Letting each state decide for itself” would, in practice, quickly become “letting each state’s judges decide for everyone.” A constitutional amendment, on the other hand, would require the approval of a far larger number of citizens to get enacted. It would have to win the votes of two-thirds of congressmen and three-quarters of state legislatures. However, it would take very few voters, in very few states, to block the amendment. But, if the amendment passes, then, it will have far more democratic legitimacy than the marriage regime that is likely to form in its absence.

He has been a fellow at the Institute of Economic Affairs in London and has appeared on various television political programs and on numerous radio talk shows. Mr. Ponnuru grew up in Kansas City and went to Princeton University.