(CNN) In a significant political shift, several top Democrats considered to be possible contenders in the 2020 presidential race are backing legislation that would decriminalize marijuana.

The shift has left Democratic operatives and marijuana legalization activists across the country saying it's difficult to imagine a debate stage in the 2020 primary race where almost all of the presidential hopefuls don't publicly back removing marijuana from the federal Controlled Substances Act.

The latest signal came when Sen. Bernie Sanders, a 2016 Democratic candidate who is widely considered a top contender for the nomination in 2020, signed onto The Marijuana Justice Act on Thursday. The bill, proposed by Sen. Cory Booker, himself a 2020 contender, would effectively end the federal crackdown on marijuana by removing the drug from the Controlled Substances Act. It was also backed by Sen. Kirsten Gillibrand, another possible 2020 hopeful.

Democratic endorsement of legalization would also be a departure from the party's 2016 position. Hillary Clinton, the party's nominee in 2016, said the federal government should allow states to legalize marijuana and called for removing the drug from the schedule 1 list, but did not go so far as to call for its removal from the Controlled Substances Act.

While Democratic operatives reject the idea that the issue will be a litmus test in the presidential contest -- the party, says a Democratic National Committee official, doesn't have litmus tests -- Democratic shifts on the issue have put pressure on every candidate to either catch up to the electorate or risk falling behind.

The shift comes amid recent movement by the Trump administration on the issue. President Donald Trump recently agreed to support protecting states that have legalized marijuana, after being pressured by Sen. Cory Gardner, a Colorado Republican. The deal comes after Attorney General Jeff Sessions signaled a desire earlier this year to go after states that have legalized marijuana by rescinding guidance from the Obama administration that had laid out a policy of non-interference.

In another step toward legalization, Senate Minority Leader Chuck Schumer threw his weight behind loosening marijuana laws on Thursday, telling Vice News that he will be introducing legislation to decriminalize the drug soon.

"The legislation is long overdue based on, you know, a bunch of different facts," Schumer said. "Ultimately, it is the right thing to do. Freedom. If smoking marijuana doesn't hurt anybody else, why shouldn't we allow people to do it and not make it criminal."

A watershed moment

Activists who have long hoped the issue would become a key plank in a Democratic nomination fight are hopeful Thursday's news signals a watershed moment is on the horizon.

"Descheduling marijuana should and will likely be a litmus test in the 2020 Democratic primary," said Erik Altieri, the executive director of The National Organization for the Reform of Marijuana Laws. "As we have seen, leaders from the party are seeing that leading the charge on this is not just good policy but is good politics."

Removing marijuana from the Controlled Substances Act -- where it is now in line with heroin, LSD and ecstasy -- would be a boon for states that have backed legalization. It would allow dispensaries and growers to utilize banks like they haven't been able to before and remove the threat of a federal crackdown, something growers and sellers are particularly worried about under Trump.

Sanders, Booker and Gillibrand are not the only possible candidates backing legalized marijuana, either.

Both Colorado Gov. John Hickenlooper and Washington Gov. Jay Inslee, two other potential candidates for the 2020 Democratic nomination, have defended the successful efforts to legalize marijuana in their respective states. Sen. Elizabeth Warren said earlier this year that she is planning to introduce legislation that protects states that have legalized marijuana from a possible crackdown. And Los Angeles Mayor Eric Garcetti, who has already started to make trips out to early presidential primary and caucus states, helped implement the plan to legalize marijuana in California, making his city that largest in the United States selling legal pot.

The political shift from Democratic leaders has tracked with polling, too. According to a Pew Research Center poll released earlier this year, 61% of Americans believe marijuana should be legal, more than double the 31% who supported legalization in 2000. The support from Democrats (69%) is even more overwhelming.

Skyler McKinley, the founding deputy director of Hickenlooper's Office of Marijuana Coordination, said that while the Democratic shift is good policy, it is also good politics.

"Especially on the Democratic side," McKinley said, "I don't think anyone who wants to continue the drug war on marijuana is going to be the nominee."

He added: "Imagine the debates. If a moderator just asks, 'Do you support descheduling marijuana' and a candidate says no, that's a viral ad right there."

This level of support is not a surprise to most top Democratic operatives. According to a former Clinton campaign staffer, the issue tested markedly well in focus groups with independent-leaning voters.

In one such group, the former staffer said, one woman brought up the case for legalizing marijuana, making the argument that it would create jobs and boost the economy.

"The group almost all agreed very enthusiastically," the staffer said, adding that the idea "caught on like wildfire."

One outlier in the 2020 field may be former Vice President Joe Biden, who said in 2014 that the Obama administration backed better enforcement efforts, not outright legalization.

"I think the idea of focusing significant resources on interdicting or convicting people for smoking marijuana is a waste of our resources," Biden told TIME. "That's different than [legalization]. Our policy for our Administration is still not legalization, and that is [and] continues to be our policy."

A spokesman for Biden declined to comment.

The Democratic evolution

Democratic views on marijuana have evolved significantly over the past three decades.

In 1992, Bill Clinton, then running for president, admitted that he "experimented with marijuana a time or two" when he was a Rhodes Scholar at the University of Oxford.

"I didn't like it," he said. "I didn't inhale it, and never tried it again."

Fast forward 11 years to 2003, where all candidates at a Democratic presidential debate in Boston were asked whether they had ever used marijuana.

In a sign that the politics around the issue had changed, three of the candidates said they had and Sen. Joe Lieberman apologized for never trying the drug.

"I have a reputation for giving unpopular answers at Democratic debates," he said. "I never used marijuana. Sorry!"

President Barack Obama opposed decriminalizing marijuana during the 2008 campaign but, much like the rest of the Democratic Party, evolved on the issue in his eight years as president and, in his exit interview with Rolling Stone, said the drug should be treated similar to tobacco or alcohol and raised questions about the "untenable" situation of having states with such varied laws.

The largest sea change came during Obama's presidency, though, when Colorado votes made the state the first to legalize marijuana in 2012. Although Hickenlooper, the state's governor, initially opposed the measure, he agreed to implement the publicly backed measure and, eventually, began to support legalization with federal officials.

"Hickenlooper was our opponent in Colorado," said Mason Tvert, a partner at VS Strategies who co-directed the Amendment 64 campaign in the state. "But, after it passed, he did take action to ensure it was implemented and since then he has been evolving on the issue and he has acknowledged that it hasn't caused the problems that he thought it could."

During the 2016 campaign, both candidates were pushed on the issue. Sen. Bernie Sanders said at a CNN debate in Nevada that he would back a state plan to legalize recreational marijuana after calling for an end to the federal ban in 2015. Clinton said the federal government should allow states to legalize marijuana and called for removing the drug from the schedule 1 list. She did not say whether she backed removing the marijuana from the Controlled Substances Act altogether.