We've been over this before. There is no "cost" to a tax-cut. An extension would not need to be "offset" as it'd continue an already existing revenue stream.
IMO the question that should be asked is how the leftists plan on reducing the deficit after the tax-increase further damages our economy and thus tax revenue.
Where are leftists in this country? There is the one socialist independent. Are you referring to democrats in this country which are moderate at absolute best but considered right wing by every other political system of first world countries?
