But rather than condemn the billionaire businessman for his flamboyant attacks on other Republicans, including McCain, many Trump fans here seemed to give the presidential candidate a different message: Go get 'em.
Trump admirers showed up in droves to a rally and picnic at Oskaloosa High School, eager to catch a glimpse of the reality TV star who has dominated the 2016 campaign with both his inflammatory comments and his signature unapologetic and brash persona.
Around a dozen Trump supporters -- many of whom endured the sweltering heat outside for hours -- told CNN that they were enamored by Trump's unfiltered rhetoric, as well as the stark contrast between the New York real estate magnate and other more seasoned politicians in the GOP field.
Larry Howe, a 77-year-old retired veteran from Blakesburg, said Trump's McCain comments from last weekend never bother him.
"He's honest. He tells is like it is. A lot of people don't like that but I think we need a president that will tell it like it is," Howe said.
Tammy Runnalls, a 50-year-old Trump fan from the nearby town of Eddyville, arrived at the school at 6 a.m. to get the first spot in line.
"He doesn't need anybody's money, he's not a career politician, he's not a lawyer. I'm excited because he's not like everybody else in the field," Runnalls said. She added that Trump's McCain comments "went a little overboard," but that they didn't make her less supportive of the candidate.
At a summit in Ames, Iowa, last weekend, Trump drew widespread condemnation when he quipped that McCain was a "war hero because he was captured" during the Vietnam War.
"I like people that weren't captured, OK?" Trump said.
Though Trump partly backtracked from those original comments, saying that he does in fact believe McCain is a war hero, he has not apologized to the 2008 GOP presidential nominee. He has also since gone after his GOP competitors for criticizing him for those remarks, particularly training his ire on South Carolina Sen. Lindsey Graham for calling Trump a "jackass."
In one of the most colorful moments of the 2016 cycle, Trump read out loud Graham's cell phone number on national television earlier this week, inviting his supporters to give the senator a call.
Even undecided voters here in Oskaloosa said they found Trump's bombastic style refreshing.
"He's really stirring things up. I do like that about him. I like his outspokenness," said Steve Wickenkan, 58, a custodian for the Oskaloosa schools who has previously supported President Barack Obama.
This weekend, Trump once again demonstrated that when critics come after him, he only hits back harder.
The Des Moines Register said Friday that its journalists had been denied credentials to Trump's Saturday rally because the paper's editorial board penned a scathing piece calling Trump a "feckless blowhard" candidate asking that he drop his White House bid.
Trump mocked the paper on Twitter, writing: "The the ultra liberal and seriously failing Des Moines Register is BEGGING my team for press credentials to my event in Iowa today-but they lie!"
His supporters egged him on, cheering when Trump gleefully kicked off his hourlong rally by noting that the Des Moines Register had been kept outside.
Crystal Breuklander, an employee of the Oskaloosa Municipal Water Department, said it was Trump's willingness to take on anybody that she found so appealing.
"I like that he will speak his mind and is not swayed by others," Breuklander, 40, said. "I like his open-mindedness, and he's not intimated by anybody."