Attorney General Jeff Sessions vowed to use federal law to get tough on marijuana, announcing in January he was ending Obama-era protections for the nascent pot industry in states where it is legal. Six months into his mission, he is largely going it alone.

Mr. Sessions’ own prosecutors have yet to bring federal charges against pot businesses that are abiding by state law. And fellow Republicans in Congress, with support from President Donald Trump, are promoting several bills that would protect or even expand the legal pot...