WASHINGTON — President-elect Donald Trump backs waterboarding and his pick for CIA director has called those who have done it “patriots” not “torturers.” Yet a Trump administration faces steep legal and legislative hurdles to reinstate the interrogation practice that simulates drowning.

Under a law approved last year, all government employees, including intelligence agents, must abide by Army guidelines for interrogating prisoners — guidelines that don’t permit waterboarding. Those rules are subject to review, but it’s not clear if they can be revised to allow the practice.

If the Trump administration were to try to change the law or the guidelines, the effort would run into bipartisan opposition in Congress. The most formidable obstacle there would be a fellow Republican, John McCain. The Arizona senator, who was beaten as a prisoner of war in Vietnam in the 1960s, adamantly opposes waterboarding. As chairman of the Senate Armed Services Committee, he would be well-positioned to block any attempt to revive it.

McCain has clashed before with Trump, who during the campaign claimed the former Navy pilot wasn’t a war hero because he had been captured. At a security conference in Canada last weekend, McCain indicated he was ready to take on Trump again as he begins a six-year term after winning re-election.

“I don’t give a damn what the president of the United States wants to do or anybody else wants to do,” he said. “We will not waterboard. We will not do it.”