Who does Mitt Romney consider rich? It's not an existential question but a mathematical one, and the answer could shed light on one of the big mysteries surrounding Romney's tax plan.

The Republican presidential nominee has been reluctant to provide details about how he'll achieve his campaign promise to cut individual income tax rates by 20 percent and eliminate the Alternative Minimum Tax and the estate tax -- all without adding a dime to the federal deficit.

As Bloomberg View editors have written, such a plan is unrealistic without more revenue, so Romney will undoubtedly have to increase the tax burden on middle- and upper-income earners.

Martin Feldstein, a Romney campaign adviser, essentially confirmed this last week, writing in the Wall Street Journal that Romney would limit or eliminate tax breaks for "high-income taxpayers," who he defined as taxpayers with adjusted gross incomes of $100,000 or more.

Now, $100,000 may seem like a lot of change, especially given the median income of $75,648 for a family of four. But it's not what's typically considered "high-income," a term generally reserved for taxpayers making $200,000 or more. (Note that President Barack Obama has pledged not to raise taxes on families making less than $250,000.)

Without going further down the income ladder and roping in more taxpayers, Romney runs into a math problem. As the Tax Policy Center has noted, Romney's proposal to cut tax rates across the board by 20 percent would reduce revenues by $251 billion. But there's only about $165 billion of available tax expenditures for those with incomes above $200,000. To remain "revenue neutral," Romney would have to plug that $86 billion hole -- a task made easier by taking away tax breaks for a broader share of taxpayers.

It isn't clear what a President Romney would do in terms of limiting these tax breaks. It's worth noting that Romney's running-mate, Paul Ryan, has advocated limiting tax expenditures and in April told Bloomberg View editors that -- in exchange for lower rates -- "more of your income should be subject to taxation if you're in the higher tax brackets."

With Election Day just two months away, Romney should outline exactly who he considers to be in the "higher tax brackets" so voters know whose tax burden will go up under his plan. To get the ball rolling, let's make one of the first questions at the upcoming presidential debates: "Who do you consider rich?"

(Deborah Solomon is a member of the Bloomberg View editorial board. Follow her on Twitter.)

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