Mitt Romney has two tax principles. First principle: I'm cutting taxes. Second principle: "People at the high end" should pay the same share of total taxes. Sounds simple enough. It isn't.
About that first principle: Romney wants to extend the 2001/2003 tax cuts cuts permanently and cut each marginal tax rate by a fifth. That would bring the top marginal tax rate, now at 35 percent, down to 28 percent. To make up some lost revenue, he says he'd eliminate some deductions and exemptions.
About that second principle: "People who are at the high end" stand to keep a lot of extra money when Romney cuts their tax rates by 20 percent. But they won't pay a higher share of overall taxes? Something's gotta give. Remember: About 50% of the country doesn't even pay federal income taxes, and the top 1% pays more overall federal taxes than the bottom 60% combined.
To dig deeper, here's a breakdown, via TPC, of various brackets' share of income and share of federal taxes (income, payroll, etc). The bigger dark-blue pizza slice is the bottom 80 percent. As you move clockwise, you go up the income ladder.
Let's assume that Romney's definition of the "high end" is the top 5%. I'll direct your attention to the orange, light blue, and purple pizza slices in the right graph that represent the top 5%.
In order to keep those slices the same shape (which is Romney's second principle) while massively cutting income taxes (Romney's first principle) you have only three choices. You can eliminate a lot of popular deductions among rich people, like mortgage interest, which is a tough sell for donors. Or you can say, Screw it, we're just going to shrink the size of the pie dramatically and make do with much lower revenue and higher deficits, which is a tough sell for independents. Or, somewhat complicatedly, you can have the top 5% pay the same share, concentrate your tax cut on the 80-95% of Americans, and then ask the bottom 79% to pay a higher share, which is a tough sell for 80% of the country. (Or, really, you can see all three things at the same time.)
The upshot is that Romney's simple two-principle approach to tax policy isn't so simple. And no matter what, all conceivable Romney plans violate my own first principle: Don't permanently cut taxes.