From [1]:

Mitt Romney has discussed the possibility of imposing a nationwide crackdown on undocumented aliens, a move that his leading immigration adviser believes could force more than a million people to quit the country every year.

Kris Kobach, the source of some of Romney's most controversial ideas on immigration, has told the Guardian that he has been in direct discussions with the presidential candidate about possible changes to federal policy should Romney win the Republican nomination and go on to take the White House.

The changes would see "attrition through enforcement" – the state-level clampdown pioneered by Kobach in Arizona, Alabama and several other states – extended across the entire US in an attempt to winkle undocumented workers out of the country...

..."I have advised Romney directly, and his close team around him, that attrition through enforcement has been working, that self-deportation has been observed in Arizona and Alabama, and that this really does need to be part of our national effort," he said.

Kobach added that "you could reasonably expect that in the first four years of a new administration, if attrition through enforcement were made the centrepiece of national immigration policy, you could see the illegal alien population cut in half."

Bear in mind that this is all completely up in the air: Romney may or may not propose something, and even if he does he might not follow through on it. He might just propose something and then not aggressively defend it (as has heretofore happened with his immigration ideas). Even if he does propose something, his campaign is at the Sharron Angle level as far as defending his ideas; for instance, they can't and won't defend Romney against even obviously wrong things like the Democrats and the Obama campaign's comments about his current immigration ideas [2].

If Romney does propose a nationwide version of the Arizona law, it would in effect be just for show. He might actually mean it, but without a plan in place to defend the proposal it would never succeed. The Tea Parties are incapable of defending such plans, and even if they were capable they'd be unwilling. They'd do what they're doing now: help their supposed opponents rather than take them on.

I could defend Romney's ideas using this technique, but I couldn't do much of it and it wouldn't have much of an effect since I'd get little or no help with it (and I'd have to deal with Teapartiers trying to undercut my efforts too). So, without a major change in how his campaign and his general set of supporters operate, his plan would end up failing.

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[1] guardian.co.uk/world/2012/feb/24/
kris-kobach-immigration-law-mastermind by Ed Pilkington of The Guardian

[2] An example from an official Obama campaign site:
keepinggophonest.com/
mitt-romney-is-the-gops-most-extreme-candidate-on-immigration