The Senate is expected to vote Tuesday on an amendment that would federally legalize marijuana by allowing states to regulate their own medical and recreational pot markets. But even one senator can block a vote on the amendment, which would address a longstanding conflict between federal and state law. Sen. Cory Gardner, D-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.