WASHINGTON — A lurid domestic dispute provided the unlikely backdrop for a major test of federal power on Tuesday at the Supreme Court, one a government lawyer said could affect the eradication of chemical weapons in Syria.
The case featured a rematch between the lawyers who argued the challenge to President Obama’s health care law, and again the issue was whether Congress had exceeded its constitutional authority.
The new case added treaties to the mix, and the question for the justices was whether a domestic law enacted to carry out a treaty must itself be authorized by one of the powers granted to Congress by the Constitution. A majority of the justices seemed inclined to impose limits on Congress’s ability to enact such laws when they concern matters ordinarily left to the states.
The case arose from the actions of a Pennsylvania woman, Carol A. Bond, when she learned that her husband was the father of her best friend’s child. Ms. Bond, a microbiologist, spread harmful chemicals on the friend’s car, mailbox and doorknob in late 2006 and early 2007, inflicting a minor injury on the friend.
After local authorities decided not to pursue the matter, federal prosecutors charged Ms. Bond with using unconventional weapons in violation of a law based on the Chemical Weapons Convention, a treaty concerned with terrorists and rogue states.
There seemed little dispute among the justices that the decision to press the chemical weapons charges against Ms. Bond was misguided. Justice Anthony M. Kennedy told Solicitor General Donald B. Verrilli Jr. that it “seems unimaginable that you would bring this prosecution.”
Justice Samuel A. Alito Jr. said that “if you told ordinary people that you were going to prosecute Ms. Bond for using a chemical weapon, they would be flabbergasted.”
Later, Justice Alito said he might recently have violated the same law.
“Would it shock you if I told you that a few days ago my wife and I distributed toxic chemicals to a great number of children?” he asked. The occasion was Halloween, he explained, adding that chocolate can be poisonous to dogs.
But the question before the court was not the wisdom of a particular prosecution; it was the source of Congress’s power to enact the law.
Mr. Verrilli said it followed from the federal government’s power to enter into treaties. No additional constitutional authority was required, he said.
A 1920 Supreme Court decision, Missouri v. Holland, seemed to say as much. “If the treaty is valid, there can be no dispute about the validity of the statute” enacted to fulfill it, Justice Oliver Wendell Holmes Jr. wrote for the court.
But Paul D. Clement, a lawyer for Ms. Bond, said that principle was too broad. “If the statute at issue here really does reach every malicious use of chemicals anywhere in the nation, as the government insists, then it clearly exceeds Congress’s limited and enumerated powers,” Mr. Clement said.
The court’s more liberal members, with the possible exception of Justice Stephen G. Breyer, seemed prepared to side with an expansive interpretation of federal power.
Justices Ruth Bader Ginsburg and Elena Kagan said it was significant that the language of the law closely tracked that of the treaty, the validity of which was undisputed by the parties.
But several of the court’s more conservative members said there must be limits on what may be accomplished domestically by a law carrying out obligations among nations.
Chief Justice John G. Roberts Jr. and Justice Alito said that in recent decades treaties had dealt with all manner of things, a development that the framers of the Constitution could not have anticipated. They added that it was hard to believe that any other signatory to the chemical weapons treaty would care whether Ms. Bond was prosecuted or not.
Mr. Clement agreed. “I don’t think any one of our treaty partners said: ‘Oh, my goodness. There’s been a deployment of chemical weapons in Norristown, Pa. I sure hope the United States steps up to its treaty obligations and prosecutes this horrible deployment of chemical weapons.’ ”
The case, Bond v. United States, No. 12-158, was making a return appearance at the court. In 2011, the court unanimously ruled that Ms. Bond had suffered the sort of concrete injury — a six-year prison sentence — that gave her standing to sue.
On Tuesday, Mr. Clement offered the justices a way to decide the case in his favor that did not require a major constitutional ruling. The law does not apply, he said, to the use of chemicals for a “peaceful purpose.” That phrase, he continued, could be interpreted to mean nonwarlike activities, exempting Ms. Bond’s conduct.
“I would analogize it to a situation where two scientists in Antarctica get in a fistfight,” he said. “O.K., that’s not conduct we condone, but I don’t think we’ve violated our pledge to reserve Antarctica for only peaceful purposes.”
Mr. Verrilli, to the seeming surprise of some justices, strongly urged the court not to adopt such a narrow ruling based on the text of the law, though his position made a broader loss on constitutional grounds more likely.
“One of the very things we are trying to sort out right now in Syria under the Chemical Weapons Convention is where the line is between peaceful uses and warlike uses,” Mr. Verrilli said, adding that “it would be terribly unfortunate” for the Supreme Court to complicate matters by offering a definition.