WASHINGTON — When President Richard Nixon signed the Controlled Substances Act in 1970, the federal government put marijuana in the category of the nation’s most dangerous drugs, along with LSD, heroin and mescaline.

In legal parlance, pot is a Schedule 1 drug, with a high potential for abuse and no medical purpose.

Forty-six years later, the law might soon change, as the Obama administration prepares to make what could be its biggest decision yet on marijuana.

Suspense is mounting after the U.S. Drug Enforcement Administration missed its self-imposed June 30 deadline to decide whether to reschedule the drug and recognize its potential therapeutic value. Twenty-six states already have legalized its medical use.

For Christine Gregoire, the former Democratic governor of Washington state, a decision has been a long time coming.

In 2011, she and Republican Lincoln Chafee, who was then the governor of Rhode Island, filed a 106-page petition with the DEA, arguing that the categorization of marijuana was “fundamentally wrong and should be changed.”

In an interview, Gregoire said she “naively had such high expectations” that the DEA would act long before now, but she predicted the agency will approve the rescheduling.

“To be honest with you, I’d be shocked if they didn’t,” Gregoire said. “Frankly, in five years the entire world has changed in Washington state. Today we have recreational marijuana, and the Justice Department’s nowhere to be found.”

Voters in Washington state and Colorado became the first in the nation to legalize recreational marijuana in 2012, a year after the governors filed their petition.

With the Obama administration adopting a policy to “just look the other way” in states with recreational marijuana, Gregoire said it would be hard for the DEA to justify keeping marijuana on the Schedule 1 list.

Opinions differ on what exactly might happen when the DEA responds to the petition, but a move to reschedule marijuana would be a major milestone in the decades-long push to legalize pot.

Among other things, it could pave the way for pharmacies to fill marijuana prescriptions and allow universities and others to conduct more medical research.

Many pot entrepreneurs hope that Congress would respond by helping marijuana businesses, allowing them to deduct their expenses from their federal taxes and giving them access to banks so they can phase out their all-cash operations.

Some predict that rescheduling could even make it easier for marijuana users to challenge policies that allow employers to fire them for positive drug tests.

Allen St. Pierre, executive director of the National Organization for the Reform of Marijuana Laws, said the DEA’s decision would be “remarkably consequential,” adding: “It will really cast the direction one way or the other.”

To be sure, there are plenty of skeptics who doubt the DEA will change anything at all.

“I’ll believe it when I see it,” said Gregory Carter, medical director of St. Luke’s Rehabilitation Institute in Spokane, Wash., who helped write the petition.

The DEA has given no indication of how it might rule, and President Barack Obama has said that any decision to reschedule marijuana should be left to Congress.