Speaking on CBS' The Early Show, the former Vietnam POW and Republican opponent of President Barack Obama in the 2008 election, said there is no evidence that he knows of that shows the officials who approved the tactics weren't giving plausible legal advice.
This will have a "chilling effect on legal counsel," McCain said.
McCain, who was himself tortured as a U.S. soldier by his North Vietnamese captors, was a vocal critic of the Bush administration's treatment of terrorism suspects.
"To go back on a witch hunt that could last for a year or so is bad for the country," he said.
On Wednesday, McCain and Sens. Joe Lieberman, I-Conn., and Lindsey Graham, R-S.C., sent President Obama a letter expressing strong concern over his decision to leave the door open to the possible prosecution of officials who crafted controversial Bush administration policy on detainee interrogations.
"Pursuing such prosecutions would, we believe, have serious negative effects on the candor with which officials in any administration provide their best advice, and would take our country in a backward-looking direction at a time when our detainee-related challenges demand that we look forward," the letter said.