The Supreme Court appeared inclined Tuesday to limit federal prosecutors' use of a fraud law that has helped win convictions of high-profile corporate executives and public officials, or throw out the law altogether.

The justices, hearing two challenges to the honest services fraud law, seemed to be in broad agreement that the law is vague and has been used to make a crime out of mistakes, minor transgressions and mere ethical violations.

Justice Stephen Breyer said he worries that the Obama administration's reading of the law makes criminals out of vast numbers of U.S. workers, including possibly employees who read The Daily Racing Form on the job.

"There are 150 million workers in the United States. I think 140 of them would flunk the test," Breyer said.

The vagueness of the honest services statute "is the working problem here," Justice Anthony Kennedy said.

Justice Antonin Scalia called the law "a mush of language" and pointed out that federal prosecutors have used it different ways in different prosecutions. If the Justice Department can't figure out what is embraced by this law, "I don't know how you expect the average citizen to," Scalia said.

Justice Department attorney Michael R. Dreeben defended the law, saying it "covers bribes, kickbacks, and undisclosed conflicts of interest by an agent or fiduciary who takes action to further that interest."

The court is hearing three cases this term in which defendants are challenging the use of the law against them. The 28-word law makes it illegal for officials, executives and others to scheme to deprive those they serve and possibly others of "the intangible right to honest services."

Former newspaper mogul Conrad Black, who has served nearly 22 months of a 6 1/2-year prison term, and former Alaska legislator Bruce Weyhrauch, who has been indicted but not tried, are asking the court to throw out the prosecutions against them.

Donald Ayer, Weyhrauch's lawyer, said the government went after Weyhrauch with the honest services charge "because they couldn't prove their bribery case."

Miguel Estrada, representing Black, said "the statute is vague in all of its applications."

Former Enron CEO Jeffrey Skilling, convicted in 2006 on conspiracy, securities fraud, insider trading and lying to auditors involving the company's collapse in 2001, will have his case heard in spring. He also says the law is unconstitutionally vague.

The justices probably will not issue decisions in the cases argued Tuesday until after they hear Skilling's appeal.