These days, the rationale for a tax cut on rich families is even weaker than in the past. They’re doing just fine as is, thank you. Their taxes remain lower than before Reagan, and they have received giant pretax raises. Among the famed top 1 percent, inflation-adjusted average income has nearly doubled to $1.4 million in the last 25 years, according to Emmanuel Saez. The middle class and poor have done much worse, with median household income rising a mere 7 percent, to $56,500.

This concentration of income means that top earners are ever more important to the country’s fiscal condition. Cutting their taxes will worsen the long-term deficit and leave less money for programs that really do lift economic growth, like infrastructure, scientific research and education.

For all of these reasons, I would prefer taxes to increase at the top (while falling for middle- and low-income families). But I recognize that the election results make that idea implausible for now. And if you favor a smaller government than I do, you can make a real argument for an approach like Mnuchin’s: lower rates and fewer deductions.

It would simplify the tax code, meaning that people wouldn’t waste so much time and effort gaming it. Every year, each household in the top 1 percent saves more than $100,000 on average in exemptions, through benefits for mortgage interest, pensions and many other things. Reduce that number and the government could modestly reduce the top tax rate. The same approach can work for corporate taxes, making them more efficient and fairer to many industries.

What if Mnuchin’s comments are meaningless or a con — and Trump hopes to sign a deficit-busting, inequality-increasing tax plan? In that case, Democrats and fiscally conservative Republicans should fight hard against the plan and refuse any compromise.

They can make a clear case: Most Americans oppose a tax cut for the rich, polls indicate. History shows that the economic rationale is nonsense, which means it would not help the working-class voters who elected Trump. And the incoming president’s own Treasury secretary said it would not happen: “There will be no absolute tax cut for the upper class.”

It’s a simple, yes-or-no standard. Call it the Mnuchian standard. Any plan that cuts taxes for the rich falls short and deserves to fail.