U.S. Sen. Elizabeth Warren has had “encouraging” conversations with Republican leadership over a bill she and Republican Colorado Sen. Cory Gardner are crafting that would give states the power to completely legalize marijuana.

“So far, it’s been encouraging,” Warren said of conversations with Republican Senate leadership. “These are always delicate negotiations, but there’s reason to be encouraged that with the two of us working together that we can actually get a vote on the floor of the Senate and a vote over in the House.”

Last week, Warren and Gardner said they were working on a bipartisan bill that would leave the choice to legalize marijuana to the states.

“If a state has already passed laws legalizing marijuana, whether it’s medicinal or recreational, however the state wants to handle it, then the federal government doesn’t come in and interfere,” Warren said. “States for a long time have made their own determinations about alcohol, about tobacco. I think they ought to be able to do the same with marijuana.”

Earlier this year, Attorney General Jeff Sessions rescinded Obama-era guidelines that made marijuana prosecutions a lower priority. Sessions’ move allows individual U.S. attorneys to choose how they treat marijuana crimes. In Massachusetts, U.S. Attorney Andrew Lelling cast a shadow on the nascent recreational marijuana industry in the state when he said he could not guarantee businesses would be immune from prosecution.

If Warren and Gardner’s bill becomes law, Bay State businesses and consumers would in fact get that guarantee and would be able to operate without fear of a federal crackdown.

Morgan Fox, a spokesman for the Marijuana Policy Project, a national advocacy group, said he sees real momentum toward a federal solution.

“It’s a natural extension of both parties starting to have a real awakening of this issue,” he said.

Fox said a bill that stops short of outright legalization but allows states the option is the most likely scenario if a bill does pass.

“It’s probably the one that’s going to appeal the most to Republicans as well as Democrats being able to use that as an avenue to say we got something done,” Fox said.

Warren said the bill would also make it easier for researchers to study the effects of marijuana, including medical research.

“Right now, it’s very difficult to study marijuana because it’s a schedule one drug; that means it’s extraordinarily difficult to get a hold of, there’s not federal funding for it,” Warren said. “What our bill would do would open the door for more research around marijuana including the use of marijuana rather than opiates for treatment of pain.”

— jordan.graham@bostonherald.com