AMES, Iowa - Donald Trump's criticism Saturday that Sen. John McCain isn't a war hero overshadowed his rivals' quest for support among evangelical voters at an Iowa political gathering designed to showcase their conservative views.

In their appeals to more than 2,000 religious conservatives crowded into an sports arena, Trump and other White House hopefuls urged a crackdown on illegal immigration, a forceful approach to the Islamic State group that could include ground troops, and a devotion to Christian values.

Yet Trump's questioning of a respected Republican leader's war record dominated the day.

When the moderator pressed Trump over his recent remark that McCain was "a dummy" and pointed out that the Arizona Republican was a war hero, Trump said: "He's not a war hero. ... He is a war hero because he was captured. I like people who weren't captured."

The comment directed at a former prisoner of war drew some boos from the audience and criticism from several Republican rivals who took the stage later in the day. McCain's spokesman offered no comment.

Trump's outsized role in the Republican presidential primary began last month when 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 said Saturday he'd support "work permits or something like that" for immigrants in the country illegally, but only after "we bring illegal immigration under control."

Rick Santorum, a former U.S. senator from Pennsylvania, took it a step further. He called for curbs on illegal and legal immigration, suggesting that both groups of immigrants are hurting American-born workers. He specifically called for cutting legal immigration by 20 percent.

"What is in the best interests of American workers?" Santorum said.

The Republicans' position moves further away from GOP leaders' previous calls to embrace comprehensive immigration changes heading into a presidential election where Hispanic voters are expected to play a critical role.

On foreign policy, the candidates offered an aggressive approach to the Islamic State group, whose rise has become an increasing concern for American policy makers and a focus in the Republican presidential primary.

Retired neurosurgeon Ben Carson, a conservative favorite, said he was skeptical of the U.S. invasion of Iraq more than a decade ago. On the Islamic State group, however, Carson said there was a "strong likelihood" that American ground forces would be necessary to contain the threat.

"I would send ground troops if I needed ground troops in order to take the land," Carson said. "You're not going to take the land without troops."

While not addressing ground forces, Rubio charged that, "ISIS is someone we can humiliate," using an alternative acronym for the group.

"We need to subject them to high-profile humiliating defeats that we broadcast and advertise to the world," he continued. "We have won propaganda wars before."

The conversation came as evangelical voters eye their options in an extraordinarily crowded Republican presidential contest. There are already 15 high-profile contenders in the race, while two more are expected to join by month's end.

Iowa's evangelical voters traditionally hold great sway in the state caucuses, which are expected for the first week in February. Christian conservatives backed the winners of the last two caucuses, Mike Huckabee in 2008 and Rick Santorum in 2012, but neither became their party's nominee.

Former Iowa Republican Party Chairman Matt Strawn said it was likely too early for a leader to emerge among Christian conservatives.