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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.
When the problem it describes about faction splitting and party policy is being discussed in good faith, the phrase rarely gets used. It's only seen when someone wants to invoke an appeal to emotion.