(CNN)Marco Rubio once backed an overhaul of U.S. immigration laws — but now, he says such major reforms are no longer a political possibility.
Marco Rubio: Big immigration reforms impossible now
"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 Sunday 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."
But, he said, the influx of undocumented workers must be stopped first so that "this will never happen again."
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.
Democratic Sen. Claire McCaskill of Missouri laid into Rubio on Sunday in an interview on ABC's "This Week."
"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," McCaskill 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."
That opposition from the right also poses one of the biggest hurdles for his nascent run for the White House. Other Republican candidates, like Bush and Texas Gov. Rick Perry, will also face criticism during the nominating process for their support for in-state tuition for undocumented immigrants at public universities.
Rubio laid out a plan Sunday for how he'd handle immigration if he becomes president in line with what other immigration reform supporters, like former Florida Gov. Jeb Bush and South Carolina Sen. Lindsey Graham, have called for.
First, he said, he'd ask 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.
Only after that happens, he said, would he support legislation that creates a path to citizenship for undocumented immigrants already in the United States.
Those who have been in the country 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 could seek legal residency. And then, he said, they could apply for citizenship.
"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."