The question was not whether the continued civil commitment of prisoners for fear that they remain “sexually dangerous” violates due process principles. It was, rather, whether Congress had the constitutional power to authorize the practice at all for federal prisoners. State civil confinement laws are not at issue in the case.
Last year, a three-judge panel of the United States Court of Appeals for the Fourth Circuit, in Richmond, Va., unanimously ruled that none of the powers granted to Congress in the Constitution empowered it to authorize such civil commitments.
During Tuesday’s argument in the case, United States v. Comstock, No. 08-1224, Solicitor General Elena Kagan said the practice was needed “to run a criminal justice system that does not itself endanger the public.”
But she relied almost solely on a clause of the Constitution not ordinarily thought of as a source of freestanding authority, one that gives Congress the right “to make all laws which shall be necessary and proper for carrying into execution” its other powers.
Only Justice Antonin Scalia consistently disputed Ms. Kagan’s invocation of the “necessary and proper clause.”
“Necessary and proper doesn’t mean it is necessary and proper for the good of society,” he said. “It means it is necessary and proper for the execution of another power that the federal government is given by the constitution.”
But “there is no constitutional power on the part of the federal government to protect society from sexual predators,” he said, adding that state officials were up to the task.
Ms. Kagan agreed that the states should take the lead role. But she said the federal government “is a kind of backstop, so that if the state does not take responsibility and does not take custody, the federal government will ensure that the person will not be released.”
In practice, she conceded, states do not typically volunteer to take custody of federal prisoners completing their sentences who are said to be sexually dangerous.
Others justices were more sympathetic to Ms. Kagan’s argument.
Justice Samuel A. Alito Jr. said states might no longer view prisoners held in federal prisons, often elsewhere in the country, as their responsibility. When public safety is at issue, Justice Ruth Bader Ginsburg said, “the government has some responsibility, doesn’t it?”
The challenge to the civil commitment law was brought by five prisoners. The case of Graydon Comstock was typical. In November 2006, six days before Mr. Comstock was to have completed a 37-month sentence for receiving child pornography, Attorney General Alberto R. Gonzales certified that Mr. Comstock was a sexually dangerous person.
Lower courts ruled that the law under which Mr. Gonzales acted exceeded Congress’s constitutional authority. But those decisions have been stayed, and Mr. Comstock remains confined in a federal prison.
G. Alan DuBois, a lawyer for the prisoners, disputed Ms. Kagan’s characterization of the relationship between the criminal justice system and his clients’ cases.
“Civil commitment has never been thought to be part of the criminal justice system,” he said. “They are two separate spheres of government control and government authority.”
Ms. Kagan drew an analogy to continuing to quarantine people with communicable diseases after their sentences were complete. Would anyone say, Ms. Kagan asked, that Congress was powerless “to effect that kind of public safety measure?”
Justice Anthony M. Kennedy responded that Congressional power over interstate commerce would support that action. But Ms. Kagan relied on the Constitution’s commerce clause only glancingly in her argument in favor of the civil commitment law.
The Rehnquist court struck down federal laws concerning violence against women and the possession of guns near schools on the ground that Congress had exceeded its constitutional authority under the commerce clause, but it upheld a federal law regulating the medicinal use of home-grown marijuana.
Justice Scalia suggested various approaches the federal government could take to make sure the states confined dangerous people after they had served their sentences, prompting a response from Justice John Paul Stevens.
“I guess we can all think of a lot of different statutes,” Justice Stevens said. “We have to decide whether this one is constitutional.”