Some groups allied with the Moores argue that the challenged provision is similar to a wealth tax, which would apply not to the incomes of the very richest Americans, but their assets, like stock holdings, that now only get taxed when they are sold.

Democratic Sen. Ron Wyden of Oregon said a court ruling for the Moores could stymie legislation like the Billionaires Income Tax he introduced last week. “The Moore case could make it impossible to close those loopholes,” Wyden said.