WASHINGTON — Republican Sen. John McCain, a former Navy pilot and prisoner of war during Vietnam, issued a veiled criticism of President Donald Trump’s medical deferments that kept him from serving in the U.S. military during the conflict.

In an interview with C-SPAN that aired Sunday, the six-term Arizona lawmaker lamented that the military “drafted the lowest income level of America and the highest income level found a doctor that would say they had a bone spur.”

One of Trump’s five draft deferments came as a result of a physician’s letter stating he suffered from bone spurs in his feet. Trump’s presidential campaign described the issue as a temporary problem.

McCain spent 5 1/2 years as a prisoner of war after his plane was shot down over North Vietnam in 1967.

Trump derided McCain’s service in 2015, stating his fellow Republican wasn’t a “war hero” and adding “I like people who weren’t captured.”

Questioned about the comment on Monday, McCain spokeswoman Julie Tarallo said the senator was referring to “one of the great injustices of the Vietnam conflict that led to a majority of poor, undereducated and minority draftees. Senator McCain has long criticized the selective service program during the Vietnam War, which left the fighting to the less privileged.”

The tacit criticism from McCain reflected the escalating fight between Trump, who often has assailed McCain, and the 81-year-old GOP senator, who is battling brain cancer and has a reputation for speaking his mind.

Last week, in a speech in Philadelphia, McCain questioned “half-baked, spurious nationalism” in America’s foreign policy. Trump lashed out, insisting he would fight back and “it won’t be pretty.” The president also bemoaned McCain’s decisive vote this past summer in opposition to a GOP bill to dismantle Barack Obama’s Affordable Care Act, a move that caused the failure of Republican efforts to repeal and replace “Obamacare.”

That prompted McCain to retort: “I have faced tougher adversaries.”

The constant back-and-forth between the president and McCain stands as the latest skirmish between the two Republican Party heavyweights and another example of Trump tangling with GOP senators who could undermine his agenda in Congress.

Trump in recent weeks has feuded with Tennessee Sen. Bob Corker and Senate Majority Leader Mitch McConnell, although the president joined with the Kentucky senator at the White House to publicly declare they were on the same page. McCain played a consequential role in the health care debate and will be lobbied heavily to support the president’s push to overhaul the tax system.

During Trump’s presidency, McCain has questioned the president’s immigration policies and warned him against cozying up to Russian President Vladimir Putin. The senator also criticized Trump in August for saying that both white nationalists and counter protesters were responsible for violent clashes in Charlottesville, Virginia.

McCain insisted in a tweet that “there’s no moral equivalency between racists & Americans standing up to defy hate and bigotry” and the president should say so.

The senator underwent surgery in mid-July to remove a 2-inch (51-millimeter) blood clot in his brain after being diagnosed with an aggressive tumor called a glioblastoma. It’s the same type of tumor that killed Sen. Edward M. Kennedy at age 77 in 2009 and Beau Biden, son of then-Vice President Joe Biden, at 46 in 2015.