The House of Representatives could subpoena the first report—the one by Mueller to Barr that describes the special counsel’s charging decisions. President Trump would likely ask a court to halt that subpoena, arguing that executive privilege protects it from disclosure. That argument is tricky; executive privilege is not a blanket defense for any president. It must bend to other public interests, including criminal investigations. Both Nixon and Clinton lost that battle in court.
The bigger problem with a congressional subpoena of a confidential memo explaining Mueller’s charging decisions is a structural one. As a constitutional matter, it would set a potentially dangerous precedent by giving Congress too great a role in overseeing prosecutors’ decisions to start, pursue, and end particular criminal investigations. Congress is a political animal; enabling politically motivated criminal prosecutions is a stepping stone to tyranny.