In pulling back from Mr. Trump’s assurance during the campaign that states should be left to decide their own marijuana policies, Mr. Spicer made clear that a battle is coming over marijuana policy. It will be a fight that pits a Justice Department headed by a fervent prohibitionist, Attorney General Jeff Sessions, against the eight states — Alaska, California, Colorado, Maine, Massachusetts, Nevada, Oregon and Washington — in which voters have approved ballot measures to legally regulate marijuana, as well as other states likely to legalize marijuana in the near future.

There’s probably not much the Trump administration can do to reverse public support for legalizing marijuana. A Quinnipiac poll last Thursday found voters in the United States favored legalization by 59 percent to 36 percent, with only Republicans and older voters opposed. An impressive 71 percent, including majorities of Republicans and older voters, think the federal government should not interfere in states that have legalized marijuana. Gallup and other polls report similar findings, including strong majority support for legalizing among Republican millennials.

What Mr. Sessions’ Justice Department can do, however, is cast a chill over the rapidly growing legal and regulated marijuana industry by targeting key players with raids, seizures of property and prosecutions in federal court, and by challenging the ability of state authorities to regulate the industry. That would be shameful given the demonstrable benefits of legal regulation: tens of thousands of taxpaying jobs; hundreds of millions of dollars annually in state tax revenue; strict oversight of cultivation, product production and distribution; savings in law enforcement costs; and far fewer young people, disproportionately African-American and Latino, saddled with criminal records. Donald Trump the businessman should get this but Jeff Sessions, the reefer madness ideologue, doesn’t care.

Governors, attorneys general and legislators in the legal marijuana states (including many who initially opposed legalization) are preparing to resist federal interference in this area as in others. That is because they can see that legally regulating marijuana better advances the public safety and health objectives of state — and federal — drug policy than does persisting with ineffective prohibitionist policies.