However, the campaign just explicitly reaffirmed their claims, in very unambiguous language:
The Romney campaign wrote:The governor’s plan calls for a 20% rate cut for all brackets, revenue neutrality, while ensuring that high-income earners continue to pay at least the same share of taxes [...] All of these goals are achievable
It is true that any one (or even two) of those goals would be mathematically "achievable" but it is not true that the combination of all three is achievable ... unless you define "high earners" to include households with incomes around $100,000. But Romney has specifically rejected that definition, saying that his definition of "middle class" extends up to incomes $200,000 or $250,000.
What Romney has officially stated in very specific terms as his plan for tax cuts is mathematically impossible. In reality, he will have to break at least part of this promise, by doing one of the following:
1. Dramatically scaling back the proposed tax cut, or
2. Abandoning the promise to keep the bill revenue-neutral, resulting in much larger budget deficits, or
3. Shifting the burden of taxation away from the richest and onto households that Romney himself defines as "middle class".
My guess is that a (increasingly hypothetical) Romney Administration would choose to fudge a bit on all three of these -- scale down the tax cuts, don't fully pay for them, and shift the tax burden a bit from the rich towards the middle class.
Also, there seems to be no difference between Romney's plans and the actual record of the G.W. Bush presidency. Would Romney differ from his GOP predecessor in any detectable way? Considering that the Republicans were too embarrassed to show Bush's face at their convention -- in startling contrast to the Democrats' putting ex-President Clinton front and center -- the fact that a Romney presidency looks a lot like a re-run of the Bush presidency ought to give people pause.