When I was young, the Democratic Party included African Americans, factory workers, nerdy intellectuals, and many other diverse groups. Democrats and Republicans were roughly equally likely to be pro-choice or pro-life. In many ways, that was a healthy state of affairs. Recently, however, we have increasingly sorted into blue and red tribes, in a number of dimensions. At some point, even seemingly non-political lifestyle issues became political. President Trump recently announced that he was taking the drug hydroxychloroquine as a...