Maybe you’ve heard the warning: The country is beset by a menace. A fringe conservative minority is holding Congress hostage, extracting radical policy concessions over the will of the majority. And it’s leading the nation to fiscal, environmental and moral ruin. Maybe you haven’t heard this part: These dangerous conservatives are Democrats. “I am talking about the radical conservatives in the Democratic Party,” said Saikat Chakrabarti. “That’s who we need to counter. It’s the same across any number of issues—pay-as-you-go, free college, “Medicare for all.” These are all enormously popular in the party, but they don’t pass because of the...