Through the first five years of his presidency, Barack Obama added 11,327 pages to the Code of Federal Regulations. No one knows how many criminal laws are contained in the compendium of legal rules and regulations promulgated by executive departments and administrative agencies. When I asked the Congressional Research Service to investigate, they said they didn’t have the resources to answer.

One thing is certain, though: No matter how many laws there are, Americans are subject to them all. As John Baker, a retired law professor at Louisiana State University told the Journal in 2011, “There is no one in the United States over the age of 18 who cannot be indicted for some federal crime.”

The result is a minefield for individuals and businesses at risk of violating obscure laws or rules. Increasingly, staying out of prison and avoiding fines and legal fees depends on the arbitrary whims of a bureaucrat.

For example, the Capo family in Virginia was fined $535 in 2011 after their young daughter Skylar rescued a woodpecker. The government deemed her effort to save the bird to be taking or transporting a protected species—an illegal act according to the Federal Migratory Bird Act. The fine was later rescinded, but not before it was assessed.

Lawrence Lewis, of Washington, D.C., also felt the weight of the federal criminal code when he was charged with a felony for accidentally violating the Clean Water Act in 2007 by diverting sewage pipes to a local storm drain, which, unknown to him, emptied into the Potomac River. Mr. Lewis pleaded guilty to a misdemeanor.

Then there was Abner Schoenwetter, who spent six years in federal prison after being found guilty in 2000 of packaging lobsters with plastic, rather than cardboard—a violation of an obscure Honduran regulation. Under the Lacey Act in the U.S., it is illegal for an American citizen to violate any fish or wildlife regulation of another nation.

In 2013-14, I was the chairman of a congressional task force on over-criminalization. One of the biggest takeaways from both Republicans and Democrats on the panel was the need for a default mens rea rule. Historically, a crime has encompassed two parts—a guilty act and some awareness that the act is wrong. This awareness of wrongdoing is mens rea, literally “guilty mind” in Latin. However, the federal government increasingly imposes criminal penalties for what are known as strict-liability crimes, laws that do not require any knowledge of wrongdoing by a person who breaks them.

Many Americans and small businesses are justifiably nervous that they can get into trouble despite making every effort to do the right thing. These are precisely the law-abiding citizens that I intended to help when I introduced the Criminal Code Improvement Act in late 2015. Under the bill, if a law or regulation doesn’t require demonstrated criminal intent and the conduct at issue is not conduct that a reasonable person would deem as wrong, then the government must prove that a defendant knew the act was illegal.

Congress and federal agencies can still create strict-liability crimes, but they must do so explicitly. For example, legislatures might decide that selling alcohol to minors is a strict-liability crime, so that the onus of ensuring compliance is on those who sell alcohol. Nothing in this proposed law will prevent them from doing so. A default mens rea rule will serve as a defense against arcane regulations prohibiting conduct that no reasonable person would think was wrong—such as the foreign environmental laws incorporated into the U.S. regulatory code by the Lacey Act.

Yet President Obama opposes the reform, threatening to stop progress on broader criminal-justice-reform efforts if Congress insists on the inclusion of a default mens rea rule. The administration has characterized the measure as a Republican attempt to protect corporations and white-collar criminals. Yet the White House hasn’t showed how mens rea reform would benefit corporate actors—who, unlike ordinary citizens, have legal departments that are more likely to know the law. The administration’s real motive is to protect the Justice Department, which wants to preserve its ability to secure easy convictions.

Many in Congress recognize that this isn’t a partisan issue—and maybe that is why the Criminal Code Improvement Act is earning support from prominent Democrats such as Reps. John Conyers Jr., Sheila Jackson Lee and Bobby Scott. For decades, the size of government has grown to create regulatory and criminal systems that can ensnare innocent people who have no intent to break any law. Now comes an opportunity to discourage such government overreach. It is vital that Congress not back down.

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