PHOENIX — A group that pushed for Arizona’s 2010 medical-marijuana law is gearing up for a 2016 ballot measure to allow any adult to use the drug for recreational purposes.

Mason Tvert, spokesman for the Marijuana Policy Project, said Tuesday that his group is buoyed by last month’s Obama administration announcement the Department of Justice will not try to void voter-approved laws in Colorado and Washington making recreational use of the drug legal. That, he said, paves the way for expansion of the concept into 10 other states, including Arizona.

Tvert, a Scottsdale native, acknowledged the original medical marijuana law was approved here by only a narrow margin. But he said attitudes are changing, and he predicted the dynamics will be different three years from now.

“It will be a presidential election year and we tend to see a much larger turnout, including a lot of younger voters and others who tend to be supportive of ending marijuana prohibition,” he said.

But Maricopa County Attorney Bill Montgomery promised to do what he can to kill the initiative.

“We will not get caught flat-footed and late to the issue again,” he said, a reference to the fact that there was no organized opposition to the 2010 medical-marijuana measure. Montgomery also promised to work to “counter the misinformation relied up by the Marijuana Policy Project to mislead the public about the nature and impact of drug legalization.”

The initiative envisions treating marijuana like alcohol: Regulated by the state, available to adults and sold only at state-licensed retailers. Tvert said it could piggyback on the medical-marijuana laws that have so far allowed nearly 100 state-regulated but privately owned dispensaries to open.

He also said the Colorado law could provide a good model. It permits Colorado residents to buy up to an ounce of marijuana at any one time; those who live elsewhere can obtain up to a quarter ounce.

What spurred the move is that Department of Justice memo providing “guidance” to federal prosecutors on the issue of whether to pursue retailers in states that allow the recreational use of the drug.

In a four-page memo, James Cole, the deputy federal attorney general, acknowledged sale and possession of the drug remain a federal felonies. But Cole told prosecutors they should, in essence, leave those issues to the states when only small amounts are involved.

Cole said, though, that federal prosecutors will defer to state decisions only if there are safeguards in their programs, such as preventing the sale to minors and ensuring the drug is not diverted to other states where possession for recreational purposes remains illegal.

Tvert, who now lives in Colorado, said he believes the system there — and the one envisioned for Arizona — will meet the Department of Justice concerns.

What it will not do is satisfy state and local prosecutors.

Montgomery already has dismissed the Justice Department memo as legally meaningless. He said that as long as the federal statutes remain on the books, he cannot ignore marijuana sale or use, whether for medical or recreational purposes.

And Montgomery is trying to persuade the state Court of Appeals to void the 2010 medical marijuana law as pre-empted by federal statutes, an argument that already has been rejected by a trial court judge.

“We believe voters around the country and in Arizona are ready to adopt a more sensible policy,” Tvert said.