Sen. John McCain is firing back at President Trump after he warned the veteran senator to be careful with his criticisms or he will "fight back."

"I've faced far greater challenges than this," said the Arizona Republican, who was diagnosed with brain cancer this summer, according to a report from CNN.

McCain and Trump have been fighting since Trump's presidential campaign, with Trump notably saying that McCain isn't a war hero even though he was tortured as a prisoner of war during the Vietnam War because "I like people who weren't captured."

In response to Trump's threat, McCain had this blunt response: "I’ve faced far greater challenges than this" https://t.co/nFo24e0dDe — Dan Merica (@danmericaCNN) October 17, 2017

On Monday, while receiving the National Constitution Center's Liberty Medal, McCain called the president's "America First" policy "half-baked, spurious nationalism cooked up by people who would rather find scapegoats than solve problems."

When asked about the comments, Trump boldly warned McCain to "be careful, because at some point I fight back."