Romney's tax plan does not add up. Krugman reflects the dominant view that has surfaced about Romney's tax plan. He can't cut tax rates and achieve revenue neutrality without raising taxes on the middle class. Krugman claims that Romney must of have believed that he could get away with a plan that does not add up because Paul Ryan has been successful by doing the same thing. Perhaps the rules of the game have changed.
I posted an article below by Washington Post Editorial Board that reaches a similar conclusion. It describes the problems with Romney's plan have been the source of widespread criticism, and it exposes the weaknesses in the plan that have been supported by prominent conservative economists. Apparently, it is no longer sufficient to win a debate by finding a conservative economist with the capability of twisting data around to support nonsense. There is a large supply of economists with those skills, but there is also a good supply of economists, as well as the non-partisan Congressional Budget Office, that do not support the "new math" that requires 2+2 = 5.