Brett Kavanaugh, who joined the Supreme Court this week, and Neil Gorsuch, who was appointed last year, share a commitment to maintaining the separation of powers between Congress and the executive branch. Although leftish opponents of both nominations portrayed that commitment as a threat to enlightened federal regulation, it can also produce results that progressives welcome, as illustrated by a case the Court heard last week.

At issue was the federal Sex Offender Registration and Notification Act (SORNA), which Congress passed in 2006. SORNA prescribes fines and up to 10 years in prison for sex offenders who do not register with the states where they live or who fail to keep their information up to date.

The law devotes 21 pages to establishing "a comprehensive national system" for registering sex offenders but leaves a crucial question unresolved: Do these requirements apply to sex offenders who were convicted before the law was enacted? SORNA gives the attorney general broad authority to decide which of those half a million or so sex offenders (if any) must comply and "to prescribe rules for registration of any such sex offender."

As a judge on the U.S. Court of Appeals for the 10th Circuit, Gorsuch criticized that provision for giving the attorney general too much discretion. "If the separation of powers means anything," he wrote in a 2015 SORNA case, "it must mean that the prosecutor isn't allowed to define the crimes he gets to enforce."

It was therefore not surprising that Gorsuch last week sided with a sex offender who was challenging retroactive application of SORNA. "I'm having trouble thinking of another delegation in which this Court has ever allowed the chief prosecutor of the United States to write the criminal law for those he's going to prosecute," Gorsuch said. "We say that vague criminal laws must be stricken….What's vaguer than a blank check to the attorney general of the United States to determine who he's going to prosecute?"

Writing in Slate, Mark Joseph Stern said this case "creates a dilemma for the left, because SORNA is a truly terrible law, and its retroactivity provision deserves to be invalidated." In Stern's view, "any criminal justice reformer" should oppose retroactive application of SORNA, which "drives America's mass incarceration problem by sending rehabilitated offenders back to prison because they failed to fill out certain paperwork."

The dilemma, as Stern sees it, is that "countless federal laws," including those authorizing environmental and financial regulations, "use broad language to let agencies enact policies to carry out Congress' objectives." He worries that such laws could be at risk if the Court starts to get serious about enforcing the "nondelegation doctrine," which says Congress cannot cede its legislative powers to the executive branch.

For similar reasons, fans of federal regulation are troubled by criticism of the Court's deference to administrative agencies when interpreting "ambiguous" statutes. Gorsuch has questioned that tendency, which Kavanaugh has described as "an atextual invention" and "nothing more than a judicially orchestrated shift of power from Congress to the Executive Branch."

Stern's dilemma is based on a fear of too much consistency. But if it's dangerous to let the attorney general write the laws he enforces, isn't the same thing true of regulators, especially when breaking their rules carries criminal penalties? Both situations raise the same concerns about fair notice, accountability, and abuse of power.

You might think progressives would appreciate the principle at stake here when the man in charge of the executive branch is someone they fear and loathe. If you don't trust Donald Trump's governing instincts, shouldn't you want the courts to limit the damage he can do by limiting the discretion of the agencies he controls?

The separation of powers is not an end in itself. The aim, as John Adams explained when he drafted the Massachusetts Constitution, is "a government of laws, and not of men." The idea is especially relevant in these contentious times.

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