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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 don't have a brown envelope marked "Top Secret Evil Plans" that I swiped from the DNC offices if that's what you're after. What I do have is literally every single news piece from the last decade where the phrase came up. Every article containing the phrase invariably relates to one of a right/centrist policy, a centrist candidate, or, attempts to secure funding from billionaire donors.
I could paste a long list of URLs here but it'd basically be the same as the results page for https://duckduckgo.com/?q=democrat+purity+test+news
Exactly. No one ever accused Manchin or Lieberman of "purity testing" for refusing to support non-conservative positions. When a centrist digs in their heels and refuses to compromise, they're just "being pragmatic." When a progressive does it, they're "purity testing." "Vote blue no matter who" is a farce.