On Wednesday, April 11, 2018, POTUS Trump made an explicit public commitment -- confirmed the next day by his press secretary -- to sign and implement pending (but previously languishing) bills in Congress to protect states' rights to enact and implement marijuana reform laws: https://www.gardner.senate.gov/newsroom/press-releases/gardner-protects-colorados-legal-marijuana-industry. Such a federal law would free state-legal cannabis commerce from most threats of federal prohibition enforcement, while retaining federal marijuana prohibition cooperation with state prohibition agents in unreformed states.
There may be little chance that Congress will enact such reform-state protective legislation this year, or related fixes of anti-money laundering laws and tax laws that make cannabis commerce more dangerous (having to work with cash instead of bank-managed checking accounts) and less profitable (IRC sec. 280E's denial of ordinary business expenses as tax deductions).
Still this presidential endorsement, of a national policy allowing states to fully legalize cannabis commerce, is a milestone in the ever-accelerating evolution of marijuana policy -- from racist prohibition -- to the healthier and safer legalization policy of state and municipal regulation and taxation of adult use. The Republican Congress -- for at long as it retains power -- is unlikely to drop its embrace of racially-enforced prohibition this year, but consistent national polling demonstrates that growing numbers of their constituents are jumping the sinking ship of prohibition and supporting reform.
The most immediate and effective impact of Trump's pronouncement is its bearing on USAG Sessions' marijuana-jihad -- and the enforcement priorities of local federal prosecutors in reform states like Massachusetts -- recalling Sessions' rescission of the USAG Holder-Lynch USDOJ forbearance (the Cole Memo) in January this year, encouraging local US Attorneys to attack state-legal cannabis commerce operations independently of state law enforcement (and state support for reform policies). When a general issues orders (to tolerate state reform laws), subordinates are expected to comply, particularly when the general's new order makes more sense than a subordinate officer's (Sessions) anachronistic and dangerous contrary order (prohibition enforcement, resulting in a more pernicious and widespread black market, as legal markets are closed). Thus, while broader permanent relief for cannabis commerce remains to be enacted, Trump's "promise" -- which may be undermined by his incompetence in maintaining any consistent policy -- significantly reduces the risks of federal intervention for cannabis commerce operations in strict compliance with state reform law.
Federal prohibition isn't over, but its end is in sight. Trump's remarks definitely brighten, and draw closer, the light of cannabis freedom at the end of the prohibition tunnel. For all the harm Trump has caused, random federal acts of logic and tolerance remain possible, as this news reveals; even a stopped clock is right twice a day.