Mitten's already has flip-flopped on various controversial stances before, such as:

Abortion - Pro-choice to Pro-Life
Immigration - No explanation needed. Something along the lines of "self-deportation," etc to now almost support this.
Auto-industry bailout - Wrote an article on why the big 3 should be allowed to fail and not be bailed out. Now wants credit for the bailout.

Seriously, anybody who actually thinks Romney would be a capable leader for this country is highly delusional. I would suggest actually researching and thinking for yourselves before blindly voting/supporting for Romney because he doesn't look like Obama. Heck, it's not even about Romney anymore, the presidential race basically has come down to Obama vs. People who hate Obama. The GOP base does not care who their candidate is, they just want Obama out of the office because of propaganda by Rove and the Koch brothers. This election goes way beyond immigration and more about the future of this country.

tl;dr: Too much Obama hate, not enough looking at the bigger picture. Romney has a distinct history of flip-flopping.