Supreme Court justices on Tuesday grappled with how much power Congress can pass on to federal agencies in a case that could change the way Capitol Hill legislates.

The justices on the eight-member court heard arguments over whether Congress crossed a line in 2006 when it passed the federal Sex Offender Registration and Notification Act that was signed into law by then-President George W. Bush.

Herman Gundy, who was convicted in 2005 of raping an 11-year-old girl and then indicted in 2013 for failing to register as a sex offender in Maryland and New York, is challenging the law’s registration requirements.

He argued Congress had violated the nondelegation doctrine in allowing the U.S. attorney general to make the law retroactive.

The rarely enforced principle of administrative law prohibits Congress from transferring its legislative powers to federal agencies without an “intelligible principle” or guidance on which to base its regulations.

“SORNA’s delegation provision grants unguided power to the nation’s top prosecutor to expand the scope of criminal laws and to impose burdensome, sometimes lifetime registration requirements on hundreds of thousands of individuals,” Gundy’s attorney Sarah Baumgartel argued.

“It combines criminal law-making and executive power in precisely the way that the Constitution was designed to prohibit.”

Justice Neil Gorsuch, a member of the court’s conservative wing, appeared to agree, saying Congress gave the attorney general a “blank check” to decide how, when and who should be prosecuted.

“We say that vague criminal laws must be stricken,” Gorsuch said.

“We’ve just repeated that last term. What’s vaguer than a blank check to the Attorney General of the United States to determine who he’s going to prosecute?”

Justice Ginsburg noted that Gorsuch had just concisely stated Baumgartel’s argument, prompting the attorney to joke she should end her arguments there. 

“I’ll cede my time,” she quipped, drawing laughs from the crowd.

Ginsburg argued earlier that it was Congress not the attorney general who defined what crimes required registration, where and when an individual must register, what information they have to provide and the penalties if they fail to follow the law.

“All that is specified by Congress,” she said.

Baumgartel, however, said none of those detailed provisions applied to offenders who were convicted before the law passed in 2006.

“The attorney general was given the power both to decide whether the law applied to pre-act offenders and then how it should apply,” she said.

But Principal Deputy Solicitor General Jeffery Wall argued that Congress realized when it passed SORNA there were going to be some logistical problems bringing hundreds of thousands of people onto the federal system, so it left it up to the attorney general to work it out with the states.

“And at the end of the day, that’s really much more about implementation than it is about policy judgement,” he said.

Justice Stephen Breyer, considered part of the court’s liberal wing, agreed at one point with Gorsuch.

He said there’s a potential danger in allowing the attorney general to write the laws he will enforce “because there we risk vendetta.

But Breyer also expressed concern that siding with Gundy will open the door to more regulatory challenges, noting that one brief said there are some 300,000 regulations where delegation could be challenged.

“We’ll be a busy court for quite a while,” he said.