For the first time in generations, a major overhaul of federal drug policy appeared imminent.

During his final year in office, former President Joseph R. Biden Jr. moved to downgrade marijuana from the government’s most restrictive class of drugs.

The change would not legalize recreational cannabis under federal law, but would remove it from a category that includes heroin and make it easier for scientists to study marijuana’s therapeutic uses.

Donald J. Trump, then a candidate for president, endorsed the idea last September, shortly before the presidential election, saying it stood to “unlock the medical uses of marijuana.”

But a few months into Mr. Trump’s new term in the White House, the push to move cannabis to a less restrictive category has effectively ground to a halt.

Resistance from the Drug Enforcement Administration, which oversees the federal drug classification system, slowed the process during the Biden administration, according to leading supporters and critics of the effort, and set the stage for a time-consuminglegal fight.

So far, the Trump administration has shown no sign of seeking the looser cannabis regulations Mr. Trump, as a presidential candidate, had backed. Instead, his administration released a drug policy blueprint last month making no mention of marijuana, and nominated a critic of cannabis legalization to run the D.E.A.