What’s the strongest case for the millionaire tax? After all, federal taxes are pretty progressive already. Forty years ago, the richest 1 percent paid about 18 percent of the country's federal income taxes. Today, they pay about 40 percent. Some might say that’s quite enough.

But the most straightforward case for a millionaire tax involves some simple arithmetic. The number of million-dollar-earners in the U.S. has soared this century. According to the IRS, the number of households with an adjusted gross income greater than $1 million more than doubled between 2001 and 2014, the last year with complete data. No group has grown faster than the super-rich; the number of households earning more than $10 million grew by 144 percent.

The Growth of Million-Dollar Earners

The Atlantic | Data: IRS

Between 2001 and 2014 , income earned by millionaires grew twice as fast as income earned by the rest of the country, according to IRS data. In 2001, million-dollar earners collectively reported about $600 billion. In 2014, they reported about $1.4 trillion—yes, trillion.

Millionaires Are Making More and More Money

The Atlantic | Data: IRS

What’s more, the tax code’s progressivity actually pauses, and even reverses, at the top of the income ladder, for two reasons. First, the top tax bracket begins around $450,000. That means the last dollar earned for a typical urologist (average income: $460,000) is taxed the same as the last dollar earned by a typical Fortune 500 CEO making 10 times more than that.

Second, investment income, which accounts for a large portion of the richest Americans’ haul, is taxed at a preferential rate. As a result, the merely-rich often pay a higher tax rate than the super-duper-rich. For example, in 2011, Mitt Romney made about $14 million but paid just 15 percent of it in taxes, which is far below the average effective tax rate for an ordinary millionaire. It’s more typical for a household making just under $100,000. Why was Romney’s effective tax rate so small? Because most of his income came from investments.

There are ways to restore the progressivity of the tax code without adding a millionaire bracket, such as raising taxes on investment income or limiting high-earners’ deductions on state and local taxes and mortgage interest. But the benefit of a millionaire bracket is that it would be the most direct way to capture some of the extraordinary wealth of the rich to use it for the public benefit. (There might be political benefits, too: Cutting the mortgage interest deduction seems like an attack on homeownership; taxing millionaires might not be similarly criticized as an attack on the American dream.)

In an analysis of Bannon’s plan, the Tax Foundation calculated that the policy would raise about $26 billion per year over the next decade. But the Bannon proposal kicked in at $5 million. According to the IRS, American millionaires making up to $5 million collectively make about as much money as those earning more than $5 million. So let’s double the Tax Foundation’s estimate and assume a millionaire tax would fetch about $50 billion annually, or half a trillion dollars in the next decade.