Sen. Cory Gardner (R-Colo.) secured a major concession from the Trump administration last week. The White House said Trump will get behind legislative efforts to protect states that have legalized marijuana, even though that collides with the approach favored by Attorney General Jeff Sessions. (Jacquelyn Martin/AP)

Democrats are increasingly embracing looser restrictions on marijuana — and a growing number of high-profile Republicans are joining them, marking a shift in the politics of cannabis.

As Democrats try to regain control of Congress in the November elections and make gains in state races, some strategists are wagering that being firmly on the side of easing marijuana laws will help boost turnout among millennials, a key bloc in the party’s coalition. Many of those voters have sat out recent midterms.

While pot enthusiasts celebrated their unofficial “4/20” holiday on Friday, Senate Minority Leader Charles E. Schumer (D-N.Y.) announced his support for decriminalizing marijuana at the federal level. Echoing others who have revised their positions, Schumer said his thinking had “evolved.”

Democrats are not alone in moving toward greater acceptance of a drug once broadly seen as taboo. Schumer’s new stance came a week after a top Republican senator announced an agreement with President Trump to keep his state’s legalization of recreational marijuana protected from federal interference.

Former House speaker John A. Boehner (R-Ohio) recently joined the board of advisers for a company that cultivates and dispenses cannabis. Boehner was previously an opponent of decriminalizing marijuana.

The developments mirror a change in public opinion as well as an uptick in the number of states that have legalized some form of marijuana use. Republicans, who have long advocated protecting states’ rights, have increasingly moved to shield them from federal intervention.

As they gear up for the fall campaign, both parties are trying to energize their bases to turn out at the polls. For Democrats, who have embraced the most liberal platform in decades, marijuana reform is another issue they hope will enliven their core voters.

“This motivates young people because it’s a question of freedom, of justice,” said Sen. Brian Schatz of Hawaii, part of a younger, more liberal generation of Democratic lawmakers.

Schumer’s decision was informed in part by this belief, according to a Democrat familiar with his thinking.

“We see getting millennials to the polls in the midterms as a potential huge boon for us. And we need to give them something to affirmatively pull the lever for a Democrat in the midterms,” said this Democrat, who spoke on the condition of anonymity to describe strategy.

In a written statement announcing his decision, Schumer said “there’s no better time than the present” to decriminalize marijuana. He said his planned legislation would remove it from the list of scheduled substances under the Controlled Substances Act.

There were no immediate signs that Schumer’s legislation would gain major traction in the Senate, where Republicans have a narrow majority. A spokesman for Majority Leader Mitch McConnell (R-Ky.) said he would not make a prediction regarding a bill that hadn’t been released.

In the House, which Republicans control by a wider margin and has been more conservative than the Senate, there could be even less interest.

“Count me as still skeptical,” said Rep. Tom Cole (R-Okla.), who voiced concerns about the negative impact of marijuana use on the health of Americans. Still, Cole said there is “no question opinion has shifted on this.” And he said it could affect the makeup of the electorate.

The percentage of Americans who support legalizing marijuana is nearly double what it was in 2000, according to a Pew Research Center survey conducted in the fall.

The poll showed a partisan divide, with most Democrats favoring legalization and a majority of Republicans opposed. But younger Republicans saw legalization as much more favorable than older Republicans.

Recreational marijuana is legal for adults in nine states and the District. One of those states is Colorado, where Republican Sen. Cory Gardner secured a major concession from the Trump administration last week.

The White House said Trump will get behind legislative efforts to protect states that have legalized marijuana, even though that collides with the approach favored by Attorney General Jeff Sessions.

“I’ve said from the very beginning that the two were in a different position on this,” Gardner said this week, referring to Trump and Sessions. “I think we’ve now seen that the president was, all along, hoping to follow a states’ rights approach to this.”

Gardner, who leads the Senate GOP campaign arm, had been blocking Justice Department nominees in retaliation over Sessions’s decision in January to rescind Obama-era guidance that discouraged prosecutors from enforcing federal marijuana laws in states that allow use of the substance.

Gardner and a handful of other senators are working on legislation that would ensure the federal government couldn’t interfere with states that have legalized the substance.

Boehner, who once said he was “unalterably opposed” to decriminalizing marijuana laws, announced earlier this month he was joining the company Acreage Holdings “because my thinking on cannabis has evolved.”

“I’m convinced de-scheduling the drug is needed so we can do research, help our veterans, and reverse the opioid epidemic ravaging our communities,” he wrote on Twitter.

Legalizing marijuana in some states has given rise to lucrative industries there. This has pressured members of both parties with more traditional views on the drug to consider the impact on the local economy.

“It really has become a real business that employs real people and entrepreneurs,” said Rep. Jared Polis (D-Colo.), who helped start the bipartisan Cannabis Caucus and is running for governor.

But in both parties, there are still calls to exercise more caution when it comes to overhauling marijuana laws.

Sen. Richard Blumenthal (D-Conn.), a former federal prosecutor and state attorney general, said in an interview that he still supports a ban on marijuana but is starting to rethink that view.

“It’s hard to reconcile the federal prohibition with the states legalizing it,” Blumenthal said. “Somehow, that contradiction or conflict has to be addressed. That’s not to say the federal government should simply cave and surrender its laws. But it certainly argues for rethinking the nature of the prohibition.

Elsewhere on Capitol Hill, bipartisan efforts have popped up to make less dramatic policy changes.

Sens. Orrin G. Hatch (R-Utah) and Kamala D. Harris (D-Calif.) have pressed the Justice Department to back off efforts to curtail medical marijuana research at the Drug Enforcement Administration.

McConnell teamed up with Sen. Ron Wyden (D-Ore.) on a bill that would legalize hemp, which the Republican leader proudly noted recently was being used as fiber for concrete and home insulation and even in hemp-infused beer under a research pilot program in his home state.

David Weigel and John Wagner contributed to this report.