“ Bernie or bust” is their rallying cry, and many supporters of self-described democratic socialist Sen. Bernie Sanders, I-Vt., say that they will vote for President Trump if their candidate is denied the Democratic nomination a second time. After the 2016 election, the Cooperative Congressional Election Study found about 12% of those who voted for Sanders in the Democratic primaries voted for Trump in the general election. There are early signs that Sanders fervor is so strong that this could double in 2020, a development that could hand the White House to Trump for a second term. An Emerson College...