The Senate Judiciary Committee on Monday invited Attorney General Eric Holder to testify at a hearing next month to discuss ways to resolve conflicts between state and federal marijuana laws.

The Sept. 10 hearing will come less than a year after Colorado and Washington state legalized small amounts of marijuana for personal use. Twenty states and the District of Columbia have legalized medicinal marijuana consumption.

Committee Chairman Patrick Leahy, D-Vt., has pressed the Obama administration to determine its enforcement policy for states allowing marijuana use. The senator has asked for assurances from the administration that state officials responsible for the licensing of marijuana retailers won’t face criminal penalties for carrying out their duties under those state laws.

“It is important, especially at a time of budget constraints, to determine whether it is the best use of federal resources to prosecute the personal or medicinal use of marijuana in states that have made such consumption legal,” he said.

“I believe that these state laws should be respected. At a minimum, there should be guidance about enforcement from the federal government.”