The momentum toward marijuana legalization — already accelerated this year by a raft of state ballot measures — could get an even bigger boost if Democrats win control of the Senate.

That’s partly because the controlling party will choose the chairman of the committee that determines whether a sweeping marijuana proposal advances or dies.

The so-called CARERS Act has stalled in the Judiciary Committee under Chairman Charles E. Grassley of Iowa, who, in the past, has staunchly opposed legalized marijuana.

The Democrat next in line to take the gavel, Patrick J. Leahy, has shown more interest. He said in 2013 that federal officials should not “waste their time” prosecuting marijuana crimes in states where it is legal. He also comes from Vermont, which has led the country in legalization efforts.

Michael Collins, deputy director of national affairs at the Drug Policy Alliance, a pro-legalization group, called Grassley’s reticence to take on marijuana-related measures an “obstacle” to legalization.