The deadly Paris attacks have intensified Republican opposition to letting thousands of Syrian refugees come to the United States.

In this Friday, Aug. 14, 2015, photo, Republican presidential candidate, Sen. Marco Rubio, R-Fla. at an event in New York. (AP Photo/Tina Fineberg)

GOP presidential contender Sen. Marco Rubio on Sunday said the U.S. cannot do so because it's impossible to know whether people fleeing Syria have links to Islamic militants, an apparent shift from earlier statements in which he left open the prospect of migrants being admitted with proper vetting.

The question of admitting Syrian refugees has for months been part of the National security discussion among 2016 candidates that cuts to the heart of the American identity as a refuge.