Napolitano appeared on Fox News' "Your World with Neil Cavuto," with host Charles Payne filling in for Cavuto.
"The president is right. This is almost literally a witch hunt. They're looking for anything they can find," Napolitano said.
Earlier in the interview, Napolitano said Congress has "tools with which to do it, unfortunately for the president."
"There's a Supreme Court opinion. And there are cases following it which says basically Congress' investigatory powers are limited to bona fide federal issues," he said. "But that is a very, very broad swath and Congress can investigate anything."
The judge also said the president "can do very little about this."