I don’t think Romney cares about the potential backlash a zero or near zero effective tax rate would have for the average undecided or independent voter. I really do believe that his campaign feels they can effectively win the amount of votes they need.
What I am wondering, and this may be me being stupid or re-stating something that may be obvious to everyone else, is if Romney has something in his tax base that would piss of his base. Maybe something in the charitable donation category that is just too liberal for tea baggers? Maybe a business deal with someone who may be a rival to a super-Pac sugar daddy like Sheldon Adelson? Or maybe he is just hiding some extreme earnings hidden from the IRS in foreign tax havens.