But based on the justices’ comments, the lurid facts of the case and the odd posture in which it reached the court, the eventual decision will probably offer only limited guidance on the health care law’s prospects.
The case heard Tuesday, Bond v. United States, No. 09-1227, arose from a domestic dispute. Carol A. Bond, a Pennsylvania woman, did not take it well when she learned that her husband was the father of her best friend’s child. She promised to make her former friend’s life “a living hell,” and she drew on her skills as a microbiologist to do so.
Ms. Bond spread harmful chemicals on her friend’s car, mailbox and doorknob. The friend suffered only a minor injury.
Such matters are usually handled by the local police and prosecutors. In Ms. Bond’s case, though, federal prosecutors charged her with using unconventional weapons in violation of the Chemical Weapons Convention of 1993, a treaty concerned with terrorists and rogue states.
At the argument, Justice Samuel A. Alito Jr. suggested that Congress had gone too far. Suppose, he said, that Ms. Bond had “decided to retaliate against her former friend by pouring a bottle of vinegar in the friend’s goldfish bowl.”
“As I read this statute, Justice Alito said, “that would be a violation of this statute, potentially punishable by life imprisonment.”
Ms. Bond’s lawyer, Paul D. Clement, said that a chemical used by his client was not much more exotic than vinegar. “There is something sort of odd about the government’s theory that says that I can buy a chemical weapon at Amazon.com,” he said.
In her appeal to the federal appeals court in Philadelphia, Ms. Bond argued that Congress did not have the constitutional power to use a chemical weapons treaty to address a matter of a sort routinely handled by state authorities. She cited the 10th Amendment, which says that “the powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people.”
The appeals court ruled that Ms. Bond did not have standing to raise a 10th Amendment defense. Only states, it said, can invoke the amendment.
Federal prosecutors initially embraced that line of argument, but the Justice Department abandoned it in the Supreme Court, now saying that Ms. Bond was free to try to mount a defense based on the amendment.
Since Ms. Bond and her nominal adversary agreed on the central issue in the case, the court appointed a lawyer, Stephen R. McAllister, to argue for the position the government had disowned.
The outcome of the case on the standing point did not seem in much doubt on Tuesday.
Chief Justice John G. Roberts Jr., for instance, said it would be “pretty harsh” to forbid Ms. Bond from challenging her conviction on the ground that the law under which she was convicted exceeded Congressional authority.
But the justices struggled with two other distinctions. One was how to disentangle claims that Congress had exceeded its enumerated powers in Article I of the Constitution from ones based on the 10th Amendment. The other was whether there were at least some 10th Amendment claims that could be pressed only by states.
Justice Elena Kagan suggested that the case could be decided simply on the ground that Congress had exceeded the powers listed in Article I of the Constitution.
“Are there any peculiarly 10th Amendment claims that you’re making?” she asked Mr. Clement. He replied that Ms. Bond relied “principally” on the argument that Congress had exceeded its powers but that it was possible the 10th Amendment played a role as well.
Justice Anthony M. Kennedy added that individuals had a role to play in cases that at first blush seem to implicate only a clash between federal and state sovereignty.
“Your underlying premise,” Justice Kennedy told Mr. McAllister, “is that the individual has no interest in whether or not the state has surrendered its powers to the federal government, and I just don’t think the Constitution was framed on that theory.”