Marco Rubio warned New Hampshire voters on a cold Thursday that the Islamic State terrorist group is exploiting loopholes in the U.S. immigration system to send jihadists into the country to kill Americans.

"As someone who has access to very sensitive information, I can tell you for a fact that ISIS understands our legal immigration system and is seeking to exploit it," the Florida senator, a member of the Senate Intelligence Committee, told a packed house party in Bedford at the first of three public events over two days in the Granite State.

The senator's admitted shift in favor of a tougher immigration policy comes as he continues to trail his staunchly anti-immigration rivals, Donald Trump and Ted Cruz, for the Republican presidential nomination less than four weeks before voting begins. Rubio's support for allowing undocumented workers in the U.S. a pathway to citizenship puts him at odds with many in his party, and the issue has dogged him throughout the primary as Trump and Cruz have repeatedly labeled him a supporter of "amnesty." But recent attacks by the Islamic State in Paris and California have given Rubio ammunition to mount a new push to tighten immigration laws.

"This issue is different now. We now have radical jihadist groups that are using our immigration system against us," he said in Bedford. "They are trying to get people into this country as doctors, as students, as investors, as tourists. They've already gotten someone into this country as a fiancée. They understand that our southern border is weak, and they're trying to exploit that as well. They are radical jihadist groups. The same people who inspired the attacks in Paris, who inspired the attacks in California are trying to use our immigration system against us. This has become national security issue. And when an issue changes, so must your policies."

The recent pivot dovetails Rubio's more ominous message about a U.S. potentially on the precipice of irreversible decline, in contrast to the sunny optimism that defined his campaign through 2015. He has called for additional border security and tightening a visa waiver program that lets people with certain passports travel to the U.S. freely, an idea that has broad bipartisan support in Congress.

In Bedford, he made clear he isn't backing off his support for an eventual path to citizenship for people already in the U.S. illegally, although he said "we can't even get to that point" before securing the country and its border against potential threats.

He pressed the same message an hour later at Nashua Community College.

"When I am president, our immigration system is going to be guided by a very simple principle. If we don't know 100 percent for sure who you are, and 100 percent for sure why you are coming, you will not be allowed to enter the United States of America," he said. "We're not going to have amnesty."

When asked an open-ended question about what's happening in the Middle East, Rubio eventually returned to the subject of immigration, warning that terrorists are looking for loopholes in U.S. policies to infiltrate the country. "Apocalyptic movements don't just close shop and turn into stock brokers or car wash shops. They're going to continue to fight," he said. "Either they win or we win. When I'm president, we will win."

Rubio's harsher rhetoric was assailed as short-term political pandering by a progressive, pro-immigration advocate.

"His fall from grace is complete," said Frank Sharry, the executive director of America's Voice. "In 2013, he became a courageous champion of immigration reform and now he's lurching to the right almost as fast as Ted Cruz. Once hailed as the guy who would save the GOP from its extremist wing, he's now pandering to it."

But Rubio's message was well-received by the New Hampshire Republicans at his events, many of whom said they're deeply concerned about terrorism.

"He's right," said Richard Lack, a 58-year-old software designer based in Amherst, New Hampshire, saying he worries about Islamic State fighters sneaking in to the U.S. through the Mexican or Canadian border. "And I like the fact that he's not doing this blanket 'I'm just gonna stop everybody from coming in,'" added Lack, who is torn between Rubio and Carly Fiorina in the Republican primary.

"It's rather frightening. They (terrorists) could come to an event like this and kill people. We don't feel safe any more," said Brenda Phelan of Plymouth, New Hampshire, an undecided GOP voter who agreed with Rubio's warnings about immigration in Nashua.

Thomas Popick, a 55-year-old a small business owner in Nashua, said the issue was a recurring theme among the Republican candidates who are vying to win the Feb. 9 primary in the Granite State.

"I've been to six campaign events here in New Hampshire—for Jeb Bush, John Kasich, Chris Christie, Donald Trump, and Carly Fiorina," he said. "And at all of those events the threat from ISIS has come up, and specifically how America would protect itself and its borders. I think that's an issue that has definitely resonated here among the citizens of New Hampshire."

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