But behind closed doors, lawmakers are growing increasingly nervous that with Mueller’s rise, their committee probes may be pushed aside — especially if he chooses to pursue a broad inquiry encompassing everything from the earliest allegations of Russian hacks to the circumstances that led to the firing of former FBI director James B. Comey.

Justice Department officials said Mueller would likely conduct his investigation almost entirely independent of Congress. But congressional investigators must take pains not to bump heads with the FBI’s inquiry. Committee leaders have already been having talks with senior Justice Department officials to “deconflict” their efforts — but lawmakers worry that if Mueller widens the scope of his investigation, they may have few witnesses at their disposal.

“Certainly, I think Congress’s ability to investigate this process fully is going to be hampered,” said Sen. Ron Johnson (R-Wis.).

“How can you subpoena somebody to come to Congress when they’re under criminal investigation? You can’t,” said Graham, lamenting that “one of the biggest losers of this decision is public access.”