Meanwhile, Gardner said he is marshaling fellow lawmakers to oppose the new policy. A dozen senators met Tuesday in Gardner's office "to talk about what we need to be doing legislatively and the direction we should be pursuing in Congress on this matter," he said.
Gardner declined to identify the senators, but he said they include Democrats and Republicans who represent states that have legalized marijuana for medical and recreational purposes and states that are considering doing so.
Although marijuana is illegal under federal law, eight states and the District of Columbia have passed laws allowing recreational consumption. Pot is legal, in some form or under some circumstances, in an additional 22 states.
Sessions's directive, issued last week, overturns an Obama-era policy discouraging federal enforcement in states where marijuana is legal. The attorney general said prosecutors should use their own discretion, taking into consideration the department's limited resources, the seriousness of the crime and the deterrent effect that they could impose.
Over the past two decades, public opinion has swung dramatically toward decriminalizing pot. A survey conducted in October by the Pew Research Center found that 61 percent of U.S. adults support marijuana legalization, which is nearly double the percentage favoring it in 2000. Among Republicans overall, 43 percent were in favor — although that number reached 62 percent among GOP-leaning voters younger than 40.
"Cannabis is a totally different political issue now than it was 50 years ago," said Rep. Dana Rohrabacher (R-Calif.), a leading advocate for allowing states to decide the question for themselves. "Politically, none of the old analysis is holding true."
Among Republicans, the topic is a particularly treacherous one, because it pulls at the seams of the party's coalition — pitting social conservatives against those who stand most strongly for states' rights, libertarians and fiscal hawks who see the burgeoning marijuana industry as a major source of tax revenue.
"The attorney general is going to find out pretty quickly he's in a distinct minority, not only among the American public, but in the United States Congress," said John Hudak, deputy director of the Brookings Institution's Center for Effective Public Management and author of the 2016 book "Marijuana: A Short History."
"This is one more headache," Hudak said, "and one more issue that they do not want to be on the defensive about with the voters."
Many of the places where marijuana legalization is most popular — in California, for instance, where it has just taken effect — are also areas where Republicans are struggling the hardest to hold onto House seats.
Some insist that GOP candidates will not suffer from the new policy if they make clear that they disagree.
"Republican candidates for Congress will answer that question based on what fits their district or state," said Ralph Reed, head of the Faith and Freedom Coalition, an evangelical political group that has campaigned against legalization.
"Will it really hurt us among young voters? It could," Reed acknowledged, but he added: "You cannot ignore a federal law on the books. It breeds cynicism. It undermines respect. It undermines confidence in government itself."
How important marijuana becomes as a topic of campaign debate is likely to hinge on how Sessions's directive, issued last week, is implemented by U.S. attorneys across the country, many of whom are serving on an interim basis until permanent appointments can be made and confirmed by the Senate.
"We now have, in what I believe is a states' rights area, 93 unelected federal officials determining state decisions," Gardner said.
Early indications are that they could head in significantly different directions, reflecting different priorities.
In Colorado, where recreational marijuana use is legal, interim U.S. attorney Bob Troyer said he will continue to put his focus only on "identifying and prosecuting those who create the greatest safety threats to our communities around the state."
But in Massachusetts, where recreational sales are to begin this year under a ballot initiative passed in 2016, U.S. Attorney Andrew Lelling indicated that no pot sellers should consider themselves safe: "Deciding, in advance, to immunize a certain category of actors from federal prosecution would be to effectively amend the laws Congress has already passed, and that I will not do."
Oregon has long had a tolerant attitude toward marijuana. It decriminalized possession of less than 1 ounce in 1973. And the newly legalized recreational pot industry brought in $85 million in tax revenue last year, said state Attorney General Ellen Rosenblum.
"We have a growing, robust industry. You can't put that genie back in the bottle," Rosenblum said. "We're not freaking out. One of the keys is a good relationship with your U.S. attorney."
In Oregon's case, the acting U.S. attorney, Billy J. Williams — who is awaiting Senate confirmation to remain in that post — has indicated he will continue to pursue "shared public safety objectives, with an emphasis on stemming the overproduction of marijuana and the diversion of marijuana out of state, dismantling criminal organizations and thwarting violent crime in our communities."
During the 2016 campaign, then-candidate Donald Trump sent mixed signals on the issue.
He told Fox News host Bill O'Reilly that recreational marijuana is causing "a lot of problems out there." But as he was campaigning in Colorado, a state where recreational marijuana is legal, Trump said in an interview with Denver's KUSA: "I think it's up to the states, yeah. I'm a states person. I think it should be up to the states, absolutely."
But after Sessions announced the new policy, the White House declared that Trump was completely in agreement.
"The President believes in enforcing federal law," press secretary Sarah Huckabee Sanders said. "That would be his top priority. That is regardless of what the topic is, whether it's marijuana, or whether it's immigration, the president strongly believes we should enforce federal law."
Sari Horwitz contributed to this report.