U.S. has single-party control of the federal government. So why isn’t it decriminalizing cannabis?

It’s a conundrum all politicians will face at one point or another: walking back a hard-lined stance from years past to align with what’s right for the present day.

The challenges elected officials face for reversing or “evolving” their political views, as former President Barack Obama used to call it, can be enormous. “Flip-flopping” almost always leads to scorn from partisan opponents and sometimes even has constituents scratching their heads.

President Joe Biden is facing such a challenge with legalizing — or at least decriminalizing — cannabis. The Democrat in the Oval Office has somehow managed to avoid committing to moving the plant off the Schedule I controlled substances list, even as his party’s leaders in the House and Senate push tooth and nail for it.

Look no further than Biden’s history as a senator himself for why this has been so difficult. From the 1970s to when he left Congress to become Obama’s vice president in 2009, Biden championed more than a half-dozen “tough-on-crime” bills in response to an influx of illegal drugs into the country. Perhaps most damagingly, he’s credited with pushing forward former President Ronald Reagan’s War on Drugs policies.

The positions, especially during the 1970s and 1980s, were mainstream for Democrats at the time. But as momentum for cannabis legalization has reached a near tipping-point, Biden has offered little help.

“The president supports leaving decisions regarding legalization for recreational use up to the states,” said Jen Pskai, Biden’s press secretary. “He also supports legalizing medicinal marijuana.”

Push looks to finally be coming to shove, as Sen. Majority Leader Chuck Schumer promised to move forward with a bill to loosen federal restrictions on this plant, even without Biden’s support.

While the House has more than enough votes to pass cannabis reform laws, skepticism remains if all Democrats in the Senate are on board. While Biden would likely sign any Democrat-sponsored legislation to reach his desk, the party will likely need each of its 50 senators on board, plus Vice President Kamala Harris to break the tie, assuming no Republican senators cross the aisle.

Schumer this week set next year’s 4/20 as a target date for overhauling the archaic prohibition laws, but some speculation has reform coming long before that date.