John McCain criticizes Donald Trump's support for waterboarding

U.S. Senator John McCain took aim at Donald Trump's defence of waterboarding on Wednesday, a day after the presumptive Republican presidential candidate told an audience, "I like it a lot. I don't think it's tough enough."

"According to Geneva conventions, it's a war crime ... it doesn't work. Because if you inflict enough pain on someone that person will tell you whatever they think that you want to hear in order to make the pain stop," McCain said.

McCain, who as a U.S. Navy pilot spent five years as a prisoner of war in Vietnam, also said the the use of interrogation tactics such as waterboarding, could endanger U.S. service members abroad.

"If we torture people than what would we expect that our enemy would do to Americans that were captured?"

McCain has been a fierce opponent of tactics deemed by by Geneva Conventions as "torture." During his five years in captivity, McCain was kept in solitary confinement for two years and beaten frequently by his Viet Cong captors.

Trump has criticized U.S. leaders for being too soft on ISIS (Islamic State in Iraq and Syria) and other enemies.

"They probably think we're weak we're stupid. We don't know what we're doing. We have no leadership. You know you have to fight fire with fire," Trump said at a rally in Ohio on Tuesday.

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This isn't the first difference of opinion between McCain and the New York real estate mogul.

Trump, at the start of his campaign, famously attacked McCain's military service record. "He's not a war hero," Trump said. "He was a war hero because he was captured. I like people who weren't captured."

He reiterated his call for the return of waterboarding as an interrogation tactic while speaking ata rally Tuesday evening in St.Clairsville, Ohio, justhours after suicide bombers killed 41 people at an airport in Istanbul.

During his speech, he asked the crowd: "What do you think about waterboarding?" As those in the audience cheered, Trump said,"I like it a lot. I don't think it's tough enough."

McCain on Wednesday expressed disbelief about Trump's trajectory in the Republican presidential race.

"If you believed that Donald Trump was going to be the nominee and you believed that Bernie Sanders was going to come close, please raise your hand. If you just raised your hand, please don't drive an automobile here in the metropolitan area. You're a danger to yourself and others. You're crazy," McCain said.