Sen. John McCain John Sidney McCainGOP lobbyists worry Trump lags in K Street fundraising Mark Kelly kicks off Senate bid: ‘A mission to lift up hardworking Arizonans’ Gabbard hits back at Meghan McCain after fight over Assad MORE (R-Ariz.) took what appeared to be a swipe at President Trump during an interview about the Vietnam War on Sunday, criticizing how upper-income Americans were able to obtain a draft deferment for having a “bone spur.”

McCain made the remarks while discussing the draft on an episode of C-SPAN3’s American History TV.

“One aspect of the [Vietnam] conflict, by the way, that I will never ever countenance is that we drafted the lowest income level of America, and the highest income level found a doctor that would say that they had a bone spur,” McCain said. “That is wrong. That is wrong. If we are going to ask every American to serve, every American should serve.”

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McCain, a Vietnam veteran who was captured by North Vietnam, was referencing an ailment cited in Trump’s draft deferments.

In total, Trump received five deferments from the draft during the Vietnam War, including one due to heel spurs, bone protrusions caused by a calcium buildup.

The comments come after Trump and the senator, who has emerged as one of the president's most vociferous GOP critics, exchanged brief but heated words following a speech McCain gave last week criticizing "half-baked, spurious nationalism."

When asked about McCain's speech, Trump said: "People have to be careful, because at some point I fight back."

"I've faced greater challenged than this," McCain shot back.

It's not the first time things have gotten heated between the pair. During the presidential race, Trump went after McCain’s history of military service.

“He’s not a war hero,” Trump said in July 2015. “He was a war hero because he was captured. I like people who weren’t captured.”

McCain spent more than five years as a prisoner of war after his plane was downed by a surface-to-air missile.