Senator John McCain delivered a thinly veiled attack of President Trump's lack of military service on Sunday evening.

McCain, who has been criticized about his war hero status by Trump, seemed to retaliate while speaking about the Vietnam War in an interview aired on C-Span Sunday night.

The 81-year-old from Arizona spoke about how wealthy Americans were able to avoid being drafted into service during the battle - in which he spent five years as a prisoner of war - by having a doctor say they had a bone spur.

If that sounds familiar to anyone, that's because it is the excuse that President Trump used in one of his five deferments during Vietnam. The other four were because he was still a college student.

'One aspect of the 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 they had a bone spur,' McCain said in the interview.

'That is wrong. That is wrong. If we are going to ask every American to serve, every American should serve.'

McCain, who has been criticized about his war hero status by Trump, seemed to retaliate while speaking about the Vietnam War in an interview aired on C-Span Sunday night

The 81-year-old from Arizona spoke about how wealthy Americans were able to avoid being drafted into service during the battle - in which he spent five years as a prisoner of war - by having a doctor say they had a bone spur

The excuse that McCain, pictured right in a 1967 photo, spoke about was the one that Trump, pictured in a high school photo in 1964, used in 1968 to avoid being drafted to serve in the Vietnam War

This less-than-subtle criticism follows Trump's continuing controversy surrounding gold star families, and particularly how he allegedly told Sgt La David Johnson's widow that her husband 'knew what he signed up for,' when he went to Niger, where he was killed.

McCain and Trump have feuded since the reality television host-turned-president used an event during his presidential campaign to claim that McCain wasn't a war hero because he was captured during the Vietnam War.

Specifically, he said: 'I like people who weren't captured.'

Trump would not apologize for the remark - and now that feud has transferred into the Trump administration, with McCain becoming one of the most fervent critics of the Republican party.

McCain was one of few Republicans to vote against Trump's proposal to repeal and replace Obamacare, earning him harsh criticism from the President.

McCain and Trump have feuded since the reality television host-turned-president used an event during his presidential campaign to claim that McCain wasn't a war hero because he was captured during the Vietnam War

Trump refused to apologize for the comment, and since then has received very little praise from McCain despite belonging to the same party. McCain is pictured bottom right alongside his squadron in 1965

McCain was a prisoner of war for five years during the Vietnam War. His service and refusal to be returned home without his fellow soldiers

In 2015 it was revealed that Trump - who actually graduated high school from the New York Military Academy - received medical deferment from the war in 1968 after showing up to a physical examination with bone spurs in his feet.

In 2016, just before he was elected to office, Trump told the New York Times: 'I had a doctor that gave me a letter - a very strong letter on the heels.'

He said his condition, bone spurs, was temporary and 'not a big problem, but it was a big enough problem.'

Bone spurs are a calcium deposit that causes a bony protrusion on the underside of the heel bone. They are generally caused by a local inflammation and can inflame or injure nearby cartilage or tendons.

And though McCain's comments might seem pointed - they come with plausible deniability - as a number of wealthy Americans used the same excuse to avoid being drafted.

This would not be the first time that the Arizona Republican criticized Trump and made allusions to his 'half-baked spurious nationalism cooked up by people who would rather find scapegoats than solve problems.'

He was speaking specifically about Trump's strong support for the military and for the troops despite never serving, and for the President's criticism of athletes who kneel during the anthem.

This would not be the first time that the Arizona Republican criticized Trump and made allusions to his 'half-baked spurious nationalism.' Trump is pictured on October 18 in a meeting with the Senate Finance Committee

McCain was also one of many to criticize the way that Trump allegedly told the late Sgt La David Johnson's (pictured left) widow Myeshia (pictured right) that he 'knew what he signed up for,' during a phone call to the woman to express his condolences

The claim he made that comment was backed up by Rep Frederica Wilson (pictured), who said she was listening into the interview when he said it

McCain was also one of many to criticize the way that Trump allegedly told the late Sgt La David Johnson's widow that he 'knew what he signed up for,' during a phone call to the woman to express his condolences.

The claim he made that comment was backed up by Rep Frederica Wilson, who said she was listening into the interview when he said it. Trump has now gone after the congresswoman with a vengeance - referring to her on Twitter as 'wacky,' in an effort to dismantle her credibility.

McCain, who was recently diagnosed with a glioblastoma brain tumor, said in the C-SPAN interview that a lot of the lessons learned in the Vietnam war can be applied today.

Specifically he spoke to the need for the US to have a 'strategy and capability to win' before getting involved in a conflict.

'It really split our society in a way that we sometimes forget. Mass arrests, demonstrations, Chicago, that all of us can look back and see on C-SPAN,' McCain said.

'It was a tumultuous time and most of it was bred by the conflict.'

Since being diagnosed with the brain tumor, which carries a five-year survival rate of 10 percent, McCain has held nothing back in speaking about or acting on his opposition to Trump or his policies.