The "We're a republic, not a democracy," comes from the 1930s as an isolationist slogan to argue against getting involved with "the defense of democracies" (the FDR argument) during World War II. From there it became a quip from the John Birch Society to delegitimize a political party. I know that sounds, hyperbolic, but it's true.

Jamelle Bouie recently did yeoman work tracking down it's start.

https://www.nytimes.com/2019/08/27/opinion/aoc-crenshaw-repu...