Even as medical marijuana supporters gear up for a vote this summer, a bipartisan group of senators is pushing separate legislation that would overhaul criminal sentencing laws with an eye toward reducing some drug-related penalties.

Majority Whip Richard J. Durbin, D-Ill., along with Republican co-sponsor Mike Lee of Utah, are hoping to bring the bill (S 1410) up for a vote sometime this spring or summer. While Lee had hoped to move within the next few weeks, Durbin said it wouldn’t come up until after the Memorial Day recess.

Even if the bill makes it to the floor, it faces stiff resistance from some law enforcement organizations and Republicans wary that it could ease punishment for dangerous criminals.

The measure would sharply cut mandatory minimum prison sentences for a variety of drug offenses and would give judges more flexibility to impose lighter penalties than those currently prescribed by law. The bill also would allow imprisoned crack cocaine offenders to seek retroactive relief from the tough sentences they are serving, while creating mandatory minimum penalties for other crimes, including sexual assault offenses.

Earlier this week, a trio of senior Republicans on the Judiciary Committee — ranking member Charles E. Grassley of Iowa, Jeff Sessions of Alabama and John Cornyn of Texas — circulated a Dear Colleague letter outlining their problems with the bill.

The measure “would benefit some of the most serious and dangerous offenders in the federal system by cutting in half (or more) mandatory minimum sentences for high-level drug trafficking offenses,” the senators wrote. They added that supporters of the bill are being “highly misleading” by describing the beneficiaries of the proposal as “low-level, nonviolent” drug offenders.

Based on their letter, it seems likely that the three Republicans would try to rally opposition once the measure hits the floor.

Meanwhile, prominent outside opposition to the legislation also has formed.

A group of former high-level federal law enforcement officials — including two former attorneys general and three former administrators of the Drug Enforcement Administration — this week announced their concerns with the bill in a letter to Senate Majority Leader Harry Reid, D-Nev., and Minority Leader Mitch McConnell, R-Ky.

“We believe the American people will be ill-served by the significant reduction of sentences for federal drug trafficking crimes that involve the sale and distribution of dangerous drugs like heroin, methamphetamines and PCP,” the group, led by Republican former attorneys general William P. Barr and Michael B. Mukasey, wrote.

Advocates remain hopeful that the Senate will take up and pass the sentencing legislation, which is supported strongly by the Obama administration.