WASHINGTON — Alaska Republican Sen. Lisa Murkowski and Massachusetts Democrat Sen. Elizabeth Warren are leading an effort to urge Attorney General Jeff Sessions not to crack down on marijuana use that is legal under state laws.

Sessions, who was outspoken about his opposition to legal marijuana while in the U.S. Senate, said earlier this week that he's reviewing the more hands-off policies of President Barack Obama's administration.

And White House Press Secretary Sean Spicer said he expects the Trump administration to pursue "greater enforcement" of federal marijuana laws.

Murkowski and Warren wrote a letter to Sessions on Thursday noting Spicer's remarks, expressing concern, and asking the attorney general to clarify the Justice Department's policy on state marijuana laws. Nine additional senators signed the letter.

Eight states and the District of Columbia have legalized recreational marijuana,

"28 states have medical marijuana laws, and 21 states have decriminalized the use of marijuana," the letter said. "These voter-approved laws have been evaluated by Governors and state Attorneys General, rigorously debated by state legislatures and the communities they serve, and implemented through thoughtful processes to ensure the proper regulated production and sale of marijuana."

Murkowski did not support Alaska's initiative to legalize marijuana, but has said she believes it is the right of the state and its citizens to decide on the issue. Marijuana remains illegal under federal law.

The Obama administration issued a memorandum in 2013, known as the Cole Memorandum, named after Deputy Attorney General James Cole. It said the federal government would not challenge state marijuana laws, and would instead focus on preventing drug-related violence and transport of marijuana to states where it is illegal, and keeping drugs out of the hands of children.

"It is essential that states that have implemented any type of practical, effective marijuana policy receive immediate assurance from the DOJ that it will respect the ability of states to enforce thoughtful, sensible drug policies in ways that do not threaten the public's health and safety," Murkowski and Warren wrote in the letter.

But Spicer indicated that won't likely be the case under President Donald Trump.

"I do believe you'll see greater enforcement of it," Spicer said at a press conference last week. "Because again there's a big difference between the medical use … that's very different than the recreational use, which is something the Department of Justice will be further looking into."

Murkowski and Warren noted in the letter that Trump said during his presidential campaign legalization should be left to the states.

They noted the importance of protecting "state infrastructure, including tax revenue, small businesses, and jobs," as well as effective use of Justice Department resources and adequate health and safety regulation on marijuana.

"As Attorney General, you have the power to determine the federal government's law enforcement priorities, including how agency resources can be best utilized. We believe that the Cole Memorandum provides a strong framework for effectively utilizing the DOJ's resources and balancing the law enforcement roles of the federal government and the states," the letter said.

This is not Murkowski's first foray into attempting to influence the attorney general on marijuana. One year ago, on March 2, 2016, the senator wrote to then-Attorney General Loretta Lynch to push back on a regulation that prohibits firearms dealers from selling to marijuana users.

There is no Senate "Cannabis Caucus" akin to the group Alaska Rep. Don Young co-founded last month, but there is a staff-level marijuana policy group in which Murkowski's office participates, according to Murkowski spokesperson Karina Petersen.

The new head of the Justice Department has a long history of opposition to legal marijuana. As a Republican senator from Alabama, Sessions said it was clear to him that "good people don't smoke marijuana."