Ottumwa, Iowa (CNN) An Iowa voter here had a straightforward question Monday for Marco Rubio: "Do you support a path to citizenship for illegal aliens?"

But Rubio's answer was anything but.

"It's not a yes or no answer," Rubio said, adding that the U.S. can't deport 12 million people.

Most Republicans back work permits for undocumented immigrants, he said. Some would be deported, and others would face "significant consequences."

And eventually some can "apply" for green cards, but Rubio didn't express his preference on citizenship.

"I have personally said that I am open for them being able to apply for -- not being awarded -- apply for a green card," Rubio said before several hundred people at a community center here. "You can't apply for citizenship -- you can apply for a green card."

Pathway to citizenship

It was the latest iteration of Rubio's handling of a subject that continues to haunt his presidential ambitions. He has repeatedly backed a pathway to citizenship, including in the controversial Gang of Eight immigration bill that he co-authored in 2013. But when he speaks to Iowa voters, he takes a noticeably tougher line.

"There will be no amnesty," he says in stump speeches and in a TV ad, looking directly into the camera.

"If you are a sanctuary city, you will lose your federal funding," Rubio thundered before several hundred voters in a Marriott hotel in Coralville, Iowa, prompting roaring applause.

The shift in rhetoric comes at an urgent time for Rubio. Polls show him consistently in third place in the Iowa caucuses, a position his advisers hope he will maintain, emerging as an alternative to Ted Cruz and Donald Trump, both of whom have taken a much tougher line on the emotional issue.

But to avoid falling behind, Rubio needs to alleviate concerns from the GOP base over his handling of the immigration issue, when he worked with a bipartisan group of senators to cut a major deal that would have created a 13-year path to citizenship for millions of undocumented immigrants -- only to abandon the plan when the measure stalled in the House.

"Sen. Rubio, the only thing he's done in the Senate so far is write the amnesty bill with Chuck Schumer, and then as soon as it got too hot, he ran away and hid," New Jersey Gov. Chris Christie said Monday on Fox News. "That's not leadership and that's not what we need from a president."

To stay in a strong position in the early primary states, Rubio must maintain momentum heading into New Hampshire; falling behind other establishment favorites -- like Christie, Jeb Bush or John Kasich -- could be detrimental to his campaign.

Son of immigrants

Rubio laces the immigration issue into his stump speech, noting that as a son of Cuban immigrants who grew up in a community of migrants, he knows personally the challenges of the immigration system.

"I've lived it," he said.

"When you know what life is like in another place, you know how special this country is," Rubio told an audience of about 500 in Bettendorf, Iowa, speaking of his parents.

And to preempt concerns from voters, Rubio brings up the issue himself, saying that the debate over immigration has changed. No longer is it just about dealing with the 11-12 million people here illegally, he says, it is a national security issue, given how Islamic State militants are trying to exploit visa laws to enter the United States.

He made the same case at last week's GOP debate.

"The issue is a dramatically different issue than it was 24 months ago," Rubio said last week in South Carolina. "Twenty-four months ago, 36 months ago, you did not have a group of radical crazies named ISIS who were burning people in cages and recruiting people to enter our country legally."

Cruz, the Texas Republican, pushed back.

"Radical Islamic terrorism was not invented 24 months ago," Cruz said.

Pushed on the issue on CBS's "Face the Nation" Sunday, Rubio said it's now a "different" threat.

"We have never had a group like ISIS that has in-depth understanding of foreign immigration practices," Rubio said.

Amnesty?

Rubio continually tells voters that he will not back "amnesty" if he's president. First, he says, the United States must enforce existing immigration laws -- and beef up existing border security and visa protections. And Congress must act on these matters individually, rather than the comprehensive measure he tried to usher through in 2013.

Rubio said Monday immigrants will not be allowed into the country if they are not thoroughly vetted.

"If we don't know 100% for sure who you are, when I'm president, you're not coming into America," Rubio said to applause.

Unless he's questioned about it, Rubio doesn't address what he would do about the millions of people who were in the country illegally. He says undocumented immigrants who committed major crimes would be deported. He says undocumented immigrants who came recently to the country wouldn't be allowed to stay.

But others, after paying fines and facing "significant" punitive measures, would be allowed stay.

Eventually, he said, letting undocumented immigrants obtain a work permit and pay taxes would "be better than what we have now," something he described as a "majority" position in his party.

Sensing uncertainty in the room, Rubio quickly cautioned: "You can't do any of that right now, and you won't be able to do any of that until you prove that illegal immigration is under control."