WASHINGTON (Reuters) - Senator Rand Paul, a possible Republican presidential candidate, will introduce legislation with two Democrats that would prevent the federal government from prosecuting medical marijuana users in states where it is legal, aides said on Monday.

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While the bill’s prospects in Congress are uncertain, it could help the libertarian-leaning Kentucky senator stand out in what is shaping up to be a crowded field ahead of the 2016 presidential election.

Public opinion has shifted dramatically toward legal marijuana in recent years, and several of Paul’s potential Republican rivals have framed it as a states’ rights issue. Few have been as active on the issue as the libertarian-leaning Kentucky senator.

Paul has been an outspoken critic of the war on drugs and has said pot users should not be put in jail. He has pushed to legalize hemp, a less-potent version of the plant, for industrial purposes.

Last month, he accused former Florida governor Jeb Bush, a potential rival, of “hypocrisy” for opposing medical marijuana in Florida after admitting to pot use as a student.

Medical marijuana is now legal in 23 states and the District of Columbia, and voters have approved it for recreational use in four states and Washington, D.C.

But it remains illegal at the federal level. That disparity locks marijuana businesses out of the banking system and exposes users to arrest.

Paul’s legislation would ensure that buyers and sellers in those states would not risk federal prosecution if they are complying with state and local laws, according to congressional aides and a marijuana-advocacy group.

Democratic senators Corey Booker of New Jersey and Kirsten Gillibrand of New York also have signed on to the bill. The three lawmakers plan to discuss it in detail at a press conference on Tuesday.

A Paul spokesman did not respond to a request for comment.

Some 46 percent of Americans support full legalization of marijuana, according to Reuters/Ipsos polling. Some 58 percent of Republicans oppose legalization, while 58 percent of Democrats support it.

Bush, Texas Senator Ted Cruz, Florida Senator Marco Rubio, and Texas Governor Rick Perry have said that states should have the right to determine their own marijuana laws.

One marijuana advocate said the potential Republican candidates have been “all talk” so far.

“It’s nice to see that Senator Paul is actually doing something about it,” said Dan Riffle of the Marijuana Policy Project, a pro-legalization group.