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https://en.wikipedia.org/wiki/Thought-terminating_clich%C3%A9
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.
Purity test just means "has principles" instead of just a D next to their name.