Donald Trump is under fire from all sides of the political spectrum for calling Senator John McCain "not a war hero" and refusing to apologize for it.

The 69-year-old presidential candidate successfully united the Republican Party against him after calling McCain, who was a former Vietnam War prisoner, "not a war hero."

According to E! News, Trump was referring to the Republican senator who had been widely considered as a war hero for enduring more than five years of imprisonment after he was shot down in enemy territory.

"He's not a war hero. He's a war hero because he was captured. I like people who weren't captured," Trump told Bob Vander Plaats, an Iowa social conservative activist who was hosting a Family Leadership Council summit held at the Stephens Auditorium in Iowa State University on Saturday.

These fiery comments ignited criticisms from his fellow GOP members as well as the senator's daughter, Meghan McCain, who took to Twitter to express her horror at what she just learned.

"I can't believe what I am reading this morning. Horrified. Disgusted. There are no words," she wrote on Sunday.

"I have a brother that just returned from Afghanistan a month ago, glad he can come home to this being said about his father and his service," she posted in another Tweet.

Meanwhile, Trump's fellow candidate Jeb Bush also expressed his opinion on the matter with a Tweet, posting: "Enough with the slanderous attacks. @SenJohnMcCain and all our veterans - particularly POWs have earned our respect and admiration."

Despite this, business magnate-turned-politician still insisted to ABC News' Martha Raddatz that he would not apologize to the senator.

"When I left the room, it was a total standing ovation. It was wonderful to see. Nobody was insulted," he explained, declaring that he does not owe anyone an apology.

On Saturday's summit, the real estate tycoon said his opinion of what a hero really depends if the person fought hard on not getting caught.

"People that fought hard and weren't captured and went through a lot, they get no credit," he said, adding that it is a shame that these people were not even recognized for their accomplishment, let alone talked about.

He also expressed his disappointment at how the Republican senator did not fight for the veterans' rights in the United States considering that he was widely considered a war hero himself.

"I'm very disappointed in John McCain because the vets are horribly treated in this country. I'm fighting for the vets. I've done a lot for the vets," he said.