Phelan M. Ebenhack/AP Photo

Mitt Romney revealed Thursday that his plan to address the nation's population of 11 million illegal immigrants involves new regulation of businesses, but no real mechanism to compel the undocumented to leave.

During the Fox News debate, the former Massachusetts governor laid out more clearly than he has before details of a proposed national ID card system for legal immigrants and the requirement that employers run checks on workers or face "very serious sanctions."

Romney said the new protocols would add pressure on illegal immigrants to voluntarily return to their native countries and force them to apply to the U.S. from the "back of the line."

"You say to people who are here illegally today, you are not going to be able to work here unless you register, unless - and we will give you transition period of time, and then ultimately you have got to go home, apply for permanent residency here or citizenship, if you want to try and do that, but get in line behind everyone else," Romney said.

Immigrant advocates called the approach oversimplified and unlikely to significantly alter the status quo.

"The touch-back concept is a stimulus program for Greyhound," said Ali Noorani, executive director of the National Immigration Forum Action Fund. "The concept of getting in line is valuable and necessary, but really what's needed is the creation of a line for undocumented immigrants who are contributing to our economy."

Seventy percent of illegal immigrants have lived in the U.S. 10 years or more and many have families (with U.S. citizen children) and roots to organizations in their communities, according to the Pew Research Center.

How many of those families would uproot their lives and leave the country if Romney's plan were implemented is speculation. Romney has said, however, that he does not think government should "go through a process of tracking them all down and moving them out."

Illegal immigrants "are not going to be rounded up and boxcarred out," he told Bloomberg News in 2006.

"Romney and the other candidates are in many ways trying - advocating both sides of the coin. On the one hand they're saying they don't want to deport 11 million people, but they really want 11 million people to leave the country," said Noorani.

Mark Krikorian of the Center for Immigration Studies, an advocacy group that favors strengthened enforcement of the nation's immigration laws, said Romney's plan would be a positive step forward, even if it required some "reading between the lines."

"What he means is consistent enforcement of the law so people leave, or at least some significant share of the people leave," Krikorian said. "His point is you can't dangle amnesty in front of people if you're going to get some significant share of the population to give up and go home. And implicit in that, frankly, is that some point down the road you might talk about it."

As for how many of the 11 million might return voluntarily under a Romney policy to take a number for the 10-15 year wait for a visa, Krikorian said, "We don't know for sure, but how about let's give it a try."