So if we assume (1) that the repeal of Obamacare's tax hikes is more than paid for by repeal of Obamacare's spending and (2) that Romney could nix the exclusions interest on state and local bonds and the exclusion of inside-buildup on life insurance vehicles, then the $86 billion hole is actually a $12 billion hole. So the boost to economic growth from the Romney plan could be much smaller than the Mankiw-Weinzerl study indicates it would.

As Alex Brill writes, "if the economy were to grow just 0.1 percentage point faster per year as a result of the reform, the additional revenue in 2015 would be approximately $13 billion. The result: A $12 billion tax increase on the middle class actually becomes a tax cut."