A big tent may end up being the death of what was once the Democrat Party. Scott Jennings made the case on Tuesday as the country waited to see whether five Democratic Socialists of America-backed candidates in Missouri, Michigan, Kansas, Virginia, and Washington state would topple Democrat incumbents and push the party even further toward the radical left. What’s unfolding now is not just a primary-season skirmish; it’s the visible advance of a hostile takeover. Establishment Democrats who refuse to condemn their ideological allies, along with the party’s progressive wing, have spent months celebrating the “big tent” chief among them...