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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.
I mean you're totally right, anywhere else in the western world the Dems are a center right at best. But Trump turned the GOP into his party in less than ten years. If we on the left banded together with a common cause with deliberate action we could change the Dems similarly. But instead we'd prefer to fight ourselves.
The left keeps coming up with policy that excites voters and centrists keep kicking them to the curb and whining about those unreasonable leftists wanting to do things other than sell weapons for the genocide that centrists love so fucking dearly.