SAN FRANCISCO – A federal appeals court panel yesterday upheld the government’s authority to punish Americans for patronizing child prostitutes overseas, but one judge dissented from the ruling, warning that the law in question goes beyond Congress’s power under the Constitution to regulate foreign commerce.

The landmark 9th Circuit Court of Appeals decision and the dissent could trigger a constitutional debate that parallels the vigorous and long-standing fight over the outer bounds of the federal government’s right to legislate in areas traditionally considered the province of state and local officials.

The new ruling rejected a challenge to a law passed by Congress in 2003, the Prosecutorial Remedies and Other Tools to End the Exploitation of Children Act, or PROTECT Act. The law bans “illicit sexual conduct” overseas regardless of the purpose of the trip. Under earlier statutes, a conviction required showing that an American traveled overseas with the intent of engaging in illegal sex. Prosecutors complained that this subjective prior intent was difficult to prove beyond a reasonable doubt.

The decision from the three-judge appeals court panel involved the first person charged under the new statute, Michael Clark, 71. He was arrested in June 2003 at a Phnom Penh guest house by Cambodian officials after a French-run children’s charity received reports that he was molesting boys in the area. At the time of his arrest, Clark was engaged in sexual activity with two boys, aged 10 and 13.

Clark was turned over to American authorities and later admitted to being a pedophile and paying children for sex. He pleaded guilty to two federal felony charges. A federal judge in Seattle sentenced him to more than eight years in prison.

The two-judge majority noted that prostitution is inherently commercial, and that Congress has broad power to regulate such economic activity. “The combination of Clark’s travel in foreign commerce and his conduct of an illicit commercial sex act in Cambodia shortly thereafter puts the statute squarely within Congress’s foreign commerce Clause authority,” Judge M. Margaret McKeown wrote, in an opinion joined by Judge Procter Hug Jr.

In dissent, Judge Warren Ferguson said a paid sex act committed abroad has nothing to do with the right granted to Congress by the Constitution “to regulate commerce with foreign nations.”

“The mere act of boarding an international flight, without more, is insufficient to bring all of Clark’s downstream activities that involve an exchange of value within the ambit of Congress’s foreign commerce power,” Judge Ferguson wrote. “This cannot mean that every act with a bare economic component that occurs downstream from that travel is subject to regulation by the United States under its foreign commerce power, or the commerce clause will have been converted into a general grant of police power,” the judge wrote. “Under the interpretation of the majority, the purchase of a lunch in France by an American citizen who traveled there by airplane would constitute a constitutional act of engaging in foreign commerce.”

Judge Ferguson also noted that the law contains no “temporal link” between the travel and the alleged crime, so an American who moved overseas and never returned could be charged with an offense committed 10 or 20 years after leaving America.

The dissenting judge called Clark’s conduct “thoroughly condemnable,” but said the Constitution’s limits on congressional power must be strictly observed. “The Constitution cannot be interpreted according to the principle that the end justifies the means. The sexual abuse of children abroad is despicable, but we should not, and need not, refashion our Constitution to address it.” The judge suggested that Clark be prosecuted in Cambodia, where the offenses occurred.

Both the majority and dissenting opinions are replete with references to the Supreme Court’s recent holdings on the scope of the interstate commerce clause, including a 1995 decision, U.S. v. Lopez, which ruled unconstitutional a prosecution under the Gun Free School Zones Act, and a 2000 ruling, U.S. v. Morrison, invalidating a portion of the Violence Against Women Act. The cases have been hotly debated by legal scholars and were frequently discussed during the recent Supreme Court confirmation hearings for Judge Samuel Alito Jr.

Judge McKeown said the Supreme Court has never indicated that similar limits apply to Congress’s power to regulate foreign commerce. “There is no counterpart to Lopez or Morrison in the foreign commerce realm that would signal a retreat from the court’s expansive reading of the foreign commerce clause. In fact, the Supreme Court has never struck down an act of Congress as exceeding its powers to regulate foreign commerce,” she wrote.

A law professor at the University of California at Los Angeles, Eugene Volokh, said there are good reasons for giving Congress broad power over foreign commerce and less power over domestic activities. “In the interstate context, you have this concern about trenching on the reserved authority of the states. In the foreign context, that concern just isn’t there,” the professor said.

Mr. Volokh rejected Judge Ferguson’s argument that an American paying for sex abroad hasn’t engaged in international commerce. “That’s quintessentially commerce with a foreign nation,” he said.

The professor said Judge Ferguson may have been trying to goad his conservative or libertarian colleagues into broadening their arguments for vigorous enforcement of the commerce clause. “It’s conceivable he was trying to challenge the conservatives on the court to say if you are going to be consistent you must reach this result,” Mr. Volokh said.

Judges Ferguson and Hug were appointed by President Carter. Judge McKeown is an appointee of President Clinton.

Clark’s attorney did not return a call seeking comment yesterday. He could ask that a larger, 15-judge panel rehear the case, or that the Supreme Court take up the issue.

The law used to convict Clark also covers sex crimes committed abroad other than child prostitution, such as rape or child molestation. Whether those aspects of the law have enough of a connection to foreign commerce to be Constitutional remains an open question. Even the two judges who rejected Clark’s challenge yesterday acknowledged that a prosecution for a foreign sex crime that had no financial component would raise different issues about the scope of Congress’s authority.