"He's a war hero because he was captured," Trump said. "I like people that weren't captured."
The online response (for what it's worth) was immediate and harsh. Many pointed to the fact that Trump had received deferments that let him avoid service in Vietnam. In an ensuing press conference, Trump blamed a bone spur, among other things. A subsequent written statement offered that Trump has "great respect for all those who serve in our military including those that weren't captured and are also heroes." (And ends with: "Note, Mr. Trump left to a long lasting standing ovation, which will be by far the biggest ovation of the weekend, and much congratulatory praise.")
Unsurprisingly, his primary opponents criticized him as well, at varying degrees of severity.
As CBS' John Dickerson pointed out on Twitter, Trump's comment allows establishment Republicans who disagreed with Trump's much more vituperative comments about Mexican immigrants to at last lay into him or write him off. Several of Trump's primary opponents were loathe to respond to his immigration comments with much ferocity. But defending illegal immigrants requires a higher political investment than defending former prisoners of war. Ted Cruz, Trump's most reliable defender on the immigration remarks, declined to condemn these comments, either.
Among the conservative voters that rallied to Trump's side after his attacks on immigration, McCain is hardly a favorite. (Some laughter can be heard in the audience following Trump's comment.) But for less fervent voters who liked the idea of straight-shooting Donald Trump who was the only guy talking about immigration, the comments will likely cause some reconsideration of support. Average Republican voters support deporting illegal immigrants. They are far more likely, however, to support military service.
It's worth noting that Trump endorsed McCain during the latter's 2008 bid for the presidency. At the time, Trump said that McCain was "a man worthy of respect." He continued: "And this country no longer has respect. What we need more than anything else is just that word: Respect."