U.S. presidential candidate Donald Trump blustered his way into more trouble Saturday as his feud with the Republican Party's John McCain turned decidedly nasty, overshadowing fellow presidential hopefuls promoting their conservative credentials to evangelical Christians.

The 10 White House aspirants who converged at the annual Family Leadership Summit in Iowa offered broad support for a crackdown on illegal immigration, a forceful approach to the Islamic State group that could include ground troops, and a devotion to Christian values.

Trump overshadowed a more substantive conversation by heaping fresh criticism on McCain, an Arizona senator. It was the latest example of the reality television star's willingness to take on his own party, a practice that both excites his party's most passionate right wing and worries Republican officials.

Pressed on whether his recent criticism of McCain went too far, Trump went further.

"He is not a war hero," Trump said.

Then, when the moderator affirmed McCain was a war hero, Trump said: "He is a war hero because he was captured. I like people who weren't captured."

McCain spent more than five years in a Vietnamese prisoner of war camp after his plane was shot down during the Vietnam War.

The comment drew some boos from the audience and an even more aggressive response from national Republican leaders who fear that Trump is damaging their party's brand.

During a news conference after his appearance, Trump did not apologize but tried to clarify his remarks about McCain: "If a person is captured, they're a hero as far as I'm concerned. I don't like the job John McCain is doing in the Senate because he is not taking care of our veterans."

Trump said he avoided service in the Vietnam War through student deferments and a medical deferment, then said he didn't serve because "I was not a big fan of the Vietnam war." He added he wasn't an anti-war protester.

Earlier in the week, Trump had described McCain as "a dummy" who graduated at the bottom of his class at the U.S. Naval Academy. Trump's reaction came after McCain told an interviewer that the businessman had "fired up the crazies" with his inflammatory remarks about Mexican immigrants.

A spokesman for McCain, Brian Rogers, declined to comment when asked about Trump's latest remarks.

Trump leads Republican field

Several national polls show Trump leading the crowded Republican presidential field — with 15 candidates and counting — with former Florida Gov. Jeb Bush and Wisconsin Gov. Scott Walker also in the top tier. But despite his strong showing in the polls, Trump is still considered a long shot for the nomination.

Trump's comments about McCain drew rapid criticism from other 2016 Republican hopefuls.

Trump first got into hot water in the Republican presidential primary when he described Mexican immigrants as 'bringing drugs, they're bringing crime, they're rapists, and some, I assume, are good people.' (Lucy Nicholson/Reuters)

Former Texas governor Rick Perry said the comment makes Trump "unfit to be commander-in-chief." Bush tweeted: "Enough with the slanderous attacks. @SenJohnMcCain and all our veterans — particularly POWs have earned our respect and admiration."

Trump's outsized role in the Republican presidential primary began when, during his announcement speech last month, he described Mexican immigrants as "bringing drugs, they're bringing crime, they're rapists, and some, I assume, are good people."

"It turns out I was right," Trump declared on Saturday, citing the recent murder of a California woman by an immigrant in the country illegally. "I am so proud of the fact that I got a dialogue started on illegal immigration."

Trump was not alone in his hardline approach on illegal immigration.

Once a leading advocate for an immigration overhaul that included an eventual pathway to citizenship, Florida Sen. Marco Rubio focused almost exclusively on the need to crack down on immigrants in the country illegally as he addressed the conference.

"I don't think we can make any progress on (broader immigration reform), until we bring illegal immigration under control," Rubio said. "We have to secure our borders."

Rubio's position, like most of his party's 2016 contenders, moves further away from Republican leaders' previous calls to embrace comprehensive immigration reform heading into a presidential election where Hispanic voters are expected to play a critical role.