The Senate is expected to consider an amendment Tuesday that would federally legalize marijuana by allowing states to regulate their own medical and recreational pot markets.

But even one senator can block the amendment, which would address a longstanding conflict between federal and state law.

Sen. Cory Gardner, R-Colo., is seeking to attach the measure to the First Step Act, a bundle of prison, prisoner re-entry, and sentencing reforms that senators voted 81-12 to advance Monday.

"While we are debating criminal justice reform, we need to address the threat of prosecution by the federal government for people in Colorado that are operating legal businesses under state law," Gardner said in a statement.

But process issues could prevent a vote on Gardner's amendment, a reworking of his STATES Act, which President Trump endorsed in principle earlier this year.

"It will take unanimous consent — not Leader consent — for a vote to occur on this amendment," said David Popp, a spokesman for Senate Majority Leader Mitch McConnell, R-Ky.

It's unclear how much support the amendment has, but it's doubtful that it's unanimous. Chambers of Congress rarely vote on recreational marijuana. In a 2015 House vote, lawmakers voted 222-206 to reject an amendment from Rep. Tom McClintock, R-Calif., to protect state recreational marijuana markets from federal agents and prosecutors.

Gardner's state is one of 10 to allow adults 21 and older to use recreational marijuana. One U.S. territory and the nation's capital also allow recreational use, while more than half of states allow medical marijuana.

Since 2012, state legal recreational marijuana has boomed into a multibillion-dollar industry, though business owners technically are violating federal drug law, making risk-averse banks wary of doing business.

National polls generally show more than 60 percent support for recreational marijuana, but reform through legislation, rather than ballot initiative, is slow. New Jersey Gov. Phil Murphy, a Democrat, has struggled for a year to pass a recreational legalization law, for example.

But Congress is evolving in cannabis reformers' favor, notably after the November re-election losses of House Republicans, including House Rules Committee Chairman Rep. Pete Sessions, R-Texas, who blocked reform votes.

Last week, Congress passed a farm bill that legalizes industrial hemp, a nonintoxicating variant of cannabis. On Monday, New York Gov. Andrew Cuomo, a Democrat, said it would be a priority to legalize marijuana in his state early next year.

Update (3:15 p.m.):

Sen. Chuck Grassley, R-Iowa, objected to consideration of amendment, blocking a roll-call vote and preventing the measure's inclusion in the First Step Act.