https://en.wikipedia.org/wiki/Thought-terminating_cliché
As the party fielded more and more candidates who championed economic-right policies in order to attract corporate donations, the idea of “purity testing” was turned into a stock phrase to attack critics using labels, instead of justifying it on its own terms.
Phrases like “moral purity” now serve two functions: firstly, to dismiss criticisms from the Left wholesale without having to discuss them directly, and secondly, to blame them for the rise of fascism.
In theory it was also supposed to serve a third function of bullying the Left into voting Democrat, but that didn’t work.
What is there to work with though? There’s no cohesive movement on the left to speak of so which of the umpteen factions would they start with? We’d need to think and vote as a much bigger bloc for the left to get the kind of representation we want.