Marco Rubio once backed an overhaul of U.S. immigration laws, but now says, such major reforms are no longer a political possibility. "We can't do it in a massive piece of legislation. And I know, 'cause I tried," the Florida senator who's one week into his 2016 presidential campaign said during an appearance on CBS's "Face the Nation." Rubio said Republicans "understand that we have to deal with 12 million human beings that are in this country, that have been here for longer than a decade." The influx of undocumented workers must be stopped first so that "this will never happen again" he added. His comments come as he faces criticism from Democrats for backing off a Senate-passed overhaul of U.S. immigration law that never gained traction in the House and is anathema to conservatives.

Rubio laid out a plan and asked Congress to approve legislation requiring businesses to use the E-Verify system to check the legal status of their employees, to implement a system that tracks when foreigners with visas enter and exit the country to prevent "overstays" and to improve border security. "Once we achieve that, step two would be how we would modernize our legal immigration system" to create one that's "less family-based, more merit-based," Rubio said. "It's a long process. It's a reasonable process. It's a fair process. But it has to happen in that order," Rubio said. "And it begins with serious enforcement measures." He said, he would support the legislation only after that happens, that creates a path to citizenship for undocumented immigrants already in the United States.

Those who have been in the country for more than a decade would have to pass a background check, learn English and pay both taxes and a fine, in order to obtain a work permit, he said. After "a substantial period of time in that status," they can seek legal residency. And then, they could apply for citizenship.

"He took a principled, courageous stand on immigration reform and we passed a comprehensive bill in the Senate and then the minute his party's base started chewing on him about it, the minute Rush Limbaugh criticized him, he folded like a cheap shotgun," Democratic Sen. Claire McCaskill of Missouri said. "That's old politics. That's not what we need right now," she said. "That is the stalest trick in the book. That is shirking on your principles because of the political necessities of your party."

By Premji