Mitt Romney says “every year I’ve paid at least 13 percent [of my income in taxes] and if you add in addition the amount that goes to charity, why the number gets well above 20 percent.”

This is supposed to be in defense of not releasing his tax returns.

Assume, for the sake of the argument, he’s telling the truth. Since when are charitable contributions added to income taxes when judging whether someone has paid his fair share?

More to the point, Romney admits to an income of over $20 million a year for the last several decades. Which makes his 13 percent — or even 20 percent — violate the principle of equal sacrifice that lies at the core of our notion of tax fairness.

Even Adam Smith, the 18th century guru of free-market conservatives, saw the wisdom of a graduated tax embodying the principle of equal sacrifice. “The rich should contribute to the public expense,” he wrote, “not only in proportion to their revenue, but something more in proportion.”

Equal sacrifice means that in paying taxes people ought to feel about the same degree of pain regardless of whether they’re wealthy or poor. Logically, this means someone earning $20 million a year should pay a much larger proportion of his income in taxes than someone earning $200,000, who in turn should pay a larger proportion than someone earning $50,000.

But Romney’s alleged 13 percent tax rate is lower than that of most middle class Americans who earn a tiny fraction of what he earns.

At a time when poverty is increasing, when public parks and public libraries are being closed and when public schools are shrinking their offerings and their hours, when the nation’s debt is immense, and when the 400 richest Americans have more wealth than the bottom 150 million of us put together — Romney’s 13 percent is shameful.