The US attorney general, Jeff Sessions, is rescinding an Obama-era policy that paved the way for legalized marijuana to flourish in states across the country, creating new confusion about enforcement and use just three days after a new legalization law went into effect in California.

Instead of the previous policy of lenient federal enforcement begun under former attorney general Eric Holder in 2013, Sessions’ new stance will instead let federal prosecutors where marijuana is legal decide how aggressively to enforce longstanding federal law prohibiting it. Guidance issued on Thursday depicted the change as a “return to the rule of law”.

“It is the mission of the Department of Justice to enforce the laws of the United States, and the previous issuance of guidance undermines the rule of law and the ability of our local, state, tribal, and federal law enforcement partners to carry out this mission,” Sessions said in a statement.

Sessions’ plan drew immediate strong objection from the Republican senator Cory Gardner of Colorado, one of eight states that have legalized marijuana for recreational use.

Gardner said in a tweet that the justice department “has trampled on the will of the voters” in Colorado and other states. He said the action would contradict what Sessions had told him before the attorney general was confirmed and that he was prepared “to take all steps necessary” to fight the step including holding up the confirmation of justice department nominees.

The move by Trump’s attorney general is sure to add to confusion about whether it’s OK to grow, buy or use marijuana in states where the drug is legal. It comes just after shops opened in California, launching what is expected to become the world’s largest market for legal recreational marijuana.

“This instability will only push consumer dollars away from these state-sanctioned businesses and back into the hands of criminal elements. With nearly two-thirds of Americans, including an outright majority of Republicans, Democrats and Independents supporting marijuana legalization, this is not just bad policy, but awful politics and the Trump administration should brace itself for the public backlash it will no doubt generate,” said Erik Altieri, executive director of the National Organization for the Reform of Marijuana Laws.



Altieri also noted that the announcement throws the jobs of more than 150,000 Americans employed in the budding legal marijuana industry into limbo.

“For politicians who purport to believe in ‘small government’ and states’ rights, this is a wildly incongruous move,” said Jesselyn McCurdy, deputy director at the American Civil Liberties Union.

While Sessions has been mostly been carrying out a justice department agenda that follows Trump’s top priorities on such issues as immigration and opioids, the changes to marijuana policy reflect his own long-held concerns. Trump’s personal views on marijuana remain largely unknown.

A budtender assists a customer at the Higher Path medical marijuana dispensary in Los Angeles, California. Photograph: Robyn Beck/AFP/Getty Images

Sessions, though, has assailed marijuana as comparable to heroin and has blamed it for spikes in violence, had been expected to ramp up enforcement. His personal crusade against the substance has been well documented throughout his political career, including his over a decade in the Senate. He once famously quipped that he admired the KKK until he found out they smoked marijuana.

The Obama administration in 2013 announced it would not stand in the way of states that legalized marijuana, so long as officials acted to keep it from migrating to places where it remained outlawed and keep it out of the hands of criminal gangs and children. Sessions is rescinding that memo, written by the then deputy attorney general, James M Cole, which had cleared up some of the uncertainty about how the federal government would respond as states began allowing sales for recreational and medical purposes.

The marijuana business has since become a sophisticated, multimillion-dollar industry that helps fund some government programs. Eight states and the District of Columbia have legalized marijuana for recreational use, and California’s sales alone are projected to bring in $1bn annually in tax revenue within several years.

Sessions and some law enforcement officials in states such as Colorado blame legalization for a number of problems, including drug traffickers who have taken advantage of lax marijuana laws to illegally grow and ship the drug across state lines, where it can sell for much more. The decision was a win for marijuana opponents who had been urging Sessions to take action.

“There is no more safe haven with regard to the federal government and marijuana, but it’s also the beginning of the story and not the end,” said Kevin Sabet, president and CEO of Smart Approaches to Marijuana, who was among several anti-marijuana advocates who met with Sessions last month. “This is a victory. It’s going to dry up a lot of the institutional investment that has gone toward marijuana in the last five years.”

Threats of a federal crackdown have united liberals who object to the human costs of a war on pot with conservatives who see it as a states’ rights issue. Some in law enforcement support a tougher approach, but a bipartisan group of senators in March urged Sessions to uphold existing marijuana policy. Others in Congress have been seeking ways to protect and promote legal pot businesses.

Marijuana advocates quickly condemned Sessions’ move as a return to outdated drug-war policies that unduly affected minorities.

“It also cannot go unnoted that this policy will have a disproportionate and disastrous impact on people of color,” McCurdy said.

Although black people and white people use marijuana at similar rates, black people are almost four times more likely to be arrested for marijuana possession.

“The War on Marijuana, like the War on Drugs, has failed by almost every measure –with the exception of successfully destroying communities of color,” McCurdy added. “Today’s decision furthers entrenches the country in racially biased, fiscally irresponsible, and morally wrong drug policy.”