The recent dustup between Nancy Pelosi and the four liberal firebrands—Alexandria Ocasio-Cortez, Rashida Tlaib, Ilhan Omar, and Ayanna Pressley—in the House’s freshman class, as well as the confrontation between Kamala Harris and Joe Biden in the recent Democratic debate, reveals something troubling about the state of our politics in the US in 2019. The demand for ideological purity bodes ill for the prospects of bipartisan agreements of any kind. Not that those prospects were all that bright...