From Chuck Marr of the Center on Budget and Policy Priorities (longer version here):

Who stands to gain the most if Congress extends the middle-class [but not the top-end] Bush tax cuts: a middle-income worker or a millionaire? The millionaire….

The income tax operates as a staircase, not an elevator, so people who make $1 million a year don’t go directly to the top “floor” (i.e., to the top marginal tax rate, currently 35 percent) but instead take the “stairs,” paying tax on the first increment of taxable income at the bottom rate of 10 percent, paying tax on the next increment at 15 percent, and so on until reaching the top rate.

As a result, the 2001 tax law’s reductions in the lower tax brackets benefit not only middle-income people whose incomes fall into those lower brackets, but also people in the very highest brackets.

In fact, a family making more than $1 million will receive more than five times the tax cut benefit, in dollar terms, as a middle-class family making $50,000 to $75,000, if Congress extends the middle-class [but not the top-end] tax cuts.