Still, farmers and marijuana advocates worry: will drug agents stand on the sidelines and allow Colorado and Washington to pursue their own experiments with legalization? Or will the federal government crack down to assert its authority over drug policies?

A spokesman for the Drug Enforcement Administration in Denver said hemp farmers were “not on our radar,” but R. Gil Kerlikowske, director of the Obama administration’s Office of National Drug Control Policy, has offered stern words against both marijuana and hemp, saying that no matter what states did, the plants were still illegal in the federal government’s view.

“Hemp and marijuana are part of the same species of cannabis plant,” Mr. Kerlikowske wrote in response to a 2011 petition that sought to legalize hemp cultivation. “While most of the THC in cannabis plants is concentrated in the marijuana, all parts of the plant, including hemp, can contain THC, a Schedule I controlled substance.”

Lately, hemp has been tiptoeing toward the agricultural mainstream, gaining support from farmers’ trade groups and a wide array of politicians in statehouses and in Washington. In the Republican-controlled House, a provision tucked into the farm bill would let universities in hemp-friendly states grow small plots for research.

A handful of states, from liberal Vermont to conservative North Dakota and Kentucky, have voted to allow commercial hemp. In Vermont, any farmers who want to register as hemp growers under a new state program have to sign a form acknowledging that they risk losing their agricultural subsidies, farm equipment and livelihoods if federal agents decide to swoop in.

Every year, the federal authorities seize and destroy millions of marijuana plants — a crackdown that has rattled the medical marijuana industry in California — but the pace of seizures has dropped sharply in recent years. In 2012, federal officials reported that 3.9 million cannabis plants had been destroyed under D.E.A. eradication efforts. A year earlier, officials said they had eradicated 6.7 million plants.