In the wake of critical campaign ads by his rivals Jeb Bush and Ted Cruz, GOP White House hopeful Marco Rubio is taking a tougher stance on immigration, in an apparent attempt to bolster his conservative credentials.

The Florida senator this week voiced support for more stringent visa reviews, 20,000 additional border agents and 700 miles of new fencing, The New York Times reported. Rubio, the son of Cuban-born immigrants, has suggested that he had a change of heart after the recent terror attacks in Paris and San Bernadino, California, which he says suggest that the issue is no longer just an economic one.

Rubio: U.S. must be '100 percent sure' of immigrant motives

"It was largely about someone who came here illegally, but they're looking for a job. They're not bad people," he said about his initially moderate views on undocumented immigrants. But given that terror organizations such as ISIS now might be trying to send agents to the U.S. both legally ---- as refugees or students -- and illegally -- across the Mexican border -- the situation and Rubio's opinion have changed.

If he were to move into the White House come 2017, Rubio noted, he is committed to securing the porous U.S.-Mexican border, and registering each and every individual coming to the United States.

"If we don't know who you are, 100 percent for sure, and we're not 100 percent sure of why you're coming, we're not going to let you in," he said.

Senator still opposes massive deportations

At the same time, however, the senator reiterated on NBC's "Meet the Press" on Jan. 17 that law-abiding immigrants should be allowed to remain in the country even if they entered illegally.

"If you're a criminal alien, no, you can't stay. If you're someone that hasn't been here for a very long time, you can't stay," he said on the program. "I don't think you're gonna round up and deport 12 million people."

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