Rep. Doug Collins of Georgia, the ranking Republican on the House Judiciary Committee, put his support Monday behind a bipartisan marijuana proposal that would establish protections for states that vote to defy federal prohibition.

Mr. Collins and Rep. Suzanne Bonamici, Oregon Democrat, both signed on as co-sponsors to the Strengthening the Tenth Amendment Through Entrusting States Act of 2019, or STATES Act, bringing the bill’s total number of backers in the House to 28.

Introduced last week in both the House and Senate, passage of the bill would amend the U.S. Controlled Substances Act to provide exemptions for “any person acting in compliance with State law relating to the manufacture, production, possession, distribution, dispensation, administration or delivery” or marijuana, which has been legalized in most states for medicinal or recreational purpose despite being federally prohibited because of its status under the CSA as a Schedule 1 narcotic.

The ranking minority member of the House Judiciary Committee, Mr. Collins officially backed the bill Monday on the heels of co-signing a letter sent last week to the panel’s Democratic chairman, Rep. Jerrold Nadler of New York, asking that he “promptly hold a legislative hearing” to discuss solutions to resolve conflicting federal and state marijuana laws.

“We believe this Committee and this Congress must act to clarify the rights and responsibilities, relative to cannabis, of individuals, physicians, businesses, medical patients, and law enforcement officials,” Mr. Collins wrote in a letter Wednesday co-signed by Rep. Matt Gaetz, Florida Republican.

Mr. Nadler is not currently among the bill’s sponsors. His office did not immediately return a message seeking comment.

Thirty-three states have legalized the medicinal use of marijuana to varying degrees in the face of federal prohibition. Ten states have legalized the plant for recreational purposes, including seven that allow it to be purchased from retail shops.

Congress will consider federal marijuana reform “fairly soon,” Mr. Nadler said in late January.

The STATES Act is currently co-sponsored in the Senate by five Republicans and five Democrats.