A 'prohibitionist party' panel held Tuesday featured little balance.

The two oldest members of the U.S. Senate convened a four-witness hearing Tuesday to discuss concerns about marijuana legalization, but chose not to invite anyone supportive of the policy or with direct experience administering recreational pot laws in Western states.

Drug reform advocates denounced the hearing as a poorly informed waste of time, and senators most supportive of reforming federal marijuana laws did not attend.

Sens. Chuck Grassley, R-Iowa, and Dianne Feinstein, D-Calif., called the hearing of the Caucus on International Narcotics Control last month after a Government Accountability Office report said the Justice Department wasn’t documenting its monitoring of enforcement guidelines laid out by the Obama administration in 2013.

The guidelines were contained in a memo allowing for the first state-regulated recreational pot markets in Colorado and Washington, despite marijuana possession remaining a federal crime. There are eight enforcement...