The federal guarantee of 12 weeks off to care for children or ailing relatives should apply to every worker, the Bush administration argued Wednesday, as the Supreme Court considered scaling back a law intended to ease work and family conflicts.

The court could use the case to extend a line of rulings favoring states' rights or it could mark a detour from that legal path.

At issue is Congress' power to require that states grant time off to their own state workers, and the ability of state workers to sue if they think their leave rights were violated.

In passing the Family and Medical Leave Act in 1993, Congress said it was acting in part to stop discrimination against both women and men. Women had suffered discrimination in hiring and promotions because of assumptions they would shoulder most of the care for children or sick family, and men had suffered discrimination because they were presumed not to need time off to perform the same care, Congress reasoned.

Assistant Attorney General Viet Dinh defended the law as a practical way to counter longstanding discrimination, both overt and subtle.

"Congress sought to remedy and prevent sex-based discrimination based on assumptions about women's role in the workplace and men's role in the home," Dinh said during oral arguments at the high court.

The justices must decide if that rationale is sturdy enough to overcome state governments' usual immunity from individuals' federal lawsuits. Congress can override that immunity in limited circumstances, but a lawyer for the state of Nevada argued that this is not one of them.

There was an inadequate evidence of sex discrimination on the part of state governments, and Congress could not arbitrarily subject states to lawsuits over denied leave, Nevada Assistant Attorney General Paul Taggart argued.

"It is not fair to assume that state managers discriminated based on some stereotype," Taggart said.

Taggart seemed to find an ally in Justice Antonin Scalia, one of the court's most conservative members and a leader in the states' rights march.

"I just don't know where the government plucks 12 weeks from and says, `We've got to stop discrimination, (so) everybody gets 12 weeks,"' Scalia said. "It's an incredible leap."

In a series of cases, a narrow 5-4 majority of the high court has ruled that Congress overstepped its authority in passing various civil rights and safety laws. Taken together, the states' rights cases are considered the hallmark of the court's increasingly conservative bent under Chief Justice William H. Rehnquist.

If the court follows its recent example, it could narrow the scope of the family leave law, which allows up to 12 weeks of unpaid leave for the birth or adoption of a child or to tend to a personal or family illness.

The law applies to all workers -- the nearly 5 million people employed by state governments and the many million more who work for private companies. Protections for private workers are not at issue in the Supreme Court case, but the law's defenders said a ruling for Nevada would undermine the law nonetheless.

"The purpose of this bill was simple. You shouldn't have to choose between the family you love and the job you need," said Sen. Christopher Dodd, D-Conn., one of the measure's original supporters in Congress. "The intent of Congress was to provide basic protections for all Americans. Working for a state government shouldn't make you a second-class citizen," Dodd said.

When Congress debated the leave bill, the major opposition had nothing to do with states' rights or the Constitution. Small businesses and some other private employers feared the financial cost of allowing workers to be gone so long.

Lawyer Cornelia Pillard, representing the fired Nevada state welfare office employee at the heart of the case, argued that the law is working. The percentage of eligible men who took family leave has increased to 21 percent from 14 percent since the law was passed, she told the court.

The case is Nevada Department of Human Resources v. Hibbs, 01-1368.