A new French book that decries income inequality has become such the rage among the U.S. left that it is sparking debate on a smoldering political issue: That Hillary Clinton isn’t liberal enough to win the Democratic nomination in 2016.

Thomas Piketty‘s “Capital in the Twenty-First Century” uses a mountain of historical data to show that income inequality, a subject President Obama has seized on, will grow without government intervention. One idea is to devote up to three-quarters of income to taxes.