People gather outside the Supreme Court. (MLADEN ANTONOV/AFP/Getty Images)

A melodramatic love triangle begat a ham-handed revenge poisoning. That led to what one Supreme Court justice called an “unimaginable” federal prosecution of the scorned wife under a law enacted to implement a global chemical weapons treaty.

And that in turn led Tuesday to a grand constitutional showdown at the court that at times seemed emblematic of the nation’s long-running political debate over the limits of federal power.

At the center of the case is a question of when the federal government may intrude on powers traditionally given to the states — in this case, police powers. And a majority of the justices bristled at Solicitor General Donald B. Verrilli Jr.’s argument that courts have little place to question that intrusion when it takes the form of legislation passed by Congress to carry out a treaty.

Justice Samuel A. Alito Jr. said most people would be “flabbergasted” to know how federal prosecutors used the law targeting terrorists who use chemical weapons to go after Carol Anne Bond, a Pennsylvania microbiologist.

Bond, who was not able to have children, was outraged in 2006 when she learned that her best friend, Myrlinda Haynes, was pregnant by Bond’s husband, Clifford. Bond ordered a rare blend of chemicals, partly off the Internet, and over the next several months tried to poison Haynes 24 times by putting them on her doorknob, car and, critically, mailbox.

Haynes suffered nothing more than a burn on her fingers, and local prosecutors did not pursue charges. They suggested she call in federal officials, and postal inspectors set up surveillance that identified Bond as the assailant.

Federal prosecutors charged Bond with violating the 1998 Chemical Weapons Convention Implementation Act, a law based on the chemical weapons ban treaty that is signed by all but four of the world’s nations.

Bond pleaded guilty while reserving the right to appeal her conviction. The case has been through so many courts — including a previous stop at the Supreme Court — that she has completed her prison term (and reunited with her husband, lawyers say).

She is represented by Paul D. Clement, who was solicitor general in the George W. Bush administration and was Verrilli’s sparring partner in the court’s most recent high-profile battle over the limits of congressional power: the 2012 challenge to President Obama’s health-care law.

Clement went first and said that if the law implementing the treaty “really does reach every malicious use of chemicals anywhere in the nation, as the government insists,” then it violates the “bedrock principle of our federalist system that Congress lacks a general police power to criminalize conduct” that does not have distinctly federal concern.

Clement’s problem was that the court ruled in 1920, in a decision by Justice Oliver Wendell Holmes, that although “the great body of private relations usually fall with the control of the state, a treaty may override its power.”

Justice Ruth Bader Ginsburg said no one disputes that the treaty signed by the president and ratified by the Senate is valid and that the law required by the treaty largely follows its specifications.

“So, it’s a puzzle that the treaty could be constitutional, but the implementing legislation that adds nothing is unconstitutional,” Ginsburg said.

Clement said the difference, one that separates it from the 1920 precedent, is that the chemical weapons treaty doesn’t directly regulate individual conduct.

But Justice Elena Kagan, picking up the battle from there, told Clement that the treaty directed that legislation be passed regarding individuals.

She said the treaty gave Congress the power to pass implementing legislation. “So you have to find a constraint on the treaty power. Where does it come from?” she demanded.

Justice Sonia Sotomayor worried about the courts hamstringing efforts to deal with terrorism. “It would be deeply ironic that we have expended so much energy criticizing Syria, when if this court were now to declare that our joining or creating legislation to implement the treaty was unconstitutional,” she said.

Clement again noted the difference between countries and his client. “I don’t think any one of our treaty partners said, ‘Oh, my goodness. There’s been a deployment of chemical weapons in Norristown, Pennsylvania,’ ” he said.

He said the justices could avoid the constitutional issue by distinguishing between “war-like” uses of chemicals and vengeful acts like Bond’s.

But Verrilli, when it was his turn, said that is not up to judges. “The Framers gave the federal government exclusive control over the treaty function to ensure that it could knit the nation together as one and allow it to be fully sovereign in the conduct of foreign affairs,” he said.

Chief Justice John G. Roberts Jr., who posed no questions to Clement, asked Verrilli if it would be possible for the president to join a treaty that gives national governments all powers and for Congress then to put in place such legislation.

When Verrilli said that would be unimaginable, Justice Anthony M. Kennedy shot back: “It also seems unimaginable that you would bring this prosecution.”

That led the conservative justices — plus Justice Stephen G. Breyer, who usually sides with the liberals — to unleash a barrage of hypotheticals of what could be prosecuted under the broad law, which covers chemicals that could harm humans or animals: a wheelbarrow full of kerosene; a poisoned potato given to a horse; the performance-enhancing drugs allegedly used by cyclist Lance Armstrong.

“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?” Alito asked Verrilli, drawing laughter from the court’s spectators. He explained that chocolate Halloween candy is “poison to dogs, so it’s a toxic chemical” under the act.

Verrilli chafed, saying, “This is serious business.”

Verrilli turned down requests to identify a kind of treaty that might raise constitutional questions or to engage in the kind of line-drawing that Breyer favored.

“The Framers made a judgment that this power was going to be exclusively in the hands of the national government,” he said.

The case is Bond v. United States.