Who's against it?
I don't know. I've heard arguments from liberals that it's not a progressive tax, and I can see the point and feel the rebate covers that problem.
I think it's just not known or popular enough for people to be "against it."
"Opposition to the FairTax is bi-partisan in Washington. While most co-sponsors are Republican, those supporting the FairTax represent a minority of Republican Members. Eight years of majority rule by the GOP did not see the FairTax enacted. Democrats have been even less moved by the potential of the FairTax."