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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.
It's a new label but it's not a new concept at all. Factional bickering that weakens a broadly appealing platform has been happening since we invented partisan politics. Just on the left, look at the French Revolution and the socialist splintering in the early 20th century.
It's not a thought terminating cliche either, it's a real and tangible problem. The rise of the internet has made targeting and widening these fractures easy and effective. Any bad actor can trivially propogate a message to any number of people, making them naturally coalesce into opposing echo chambers. This chart may as well track social media use during election years.
It's true that "purity testing" is often used as a bludgeon to stifle criticism of obviously regressive policies, but it's unfair to completely ignore the kernel of truth about the opposite end of the spectrum. When a bloc of voices is lodging criticisms with no constructive platform, there's no value being added and they may as well be opposition.
It's often not that hard to tell when that line is crossed either. If your claim is candidate X is vile but you have no real option Y as a substitute, then your attack can only be strengthing the opposition. There is a political reality you have to operate in; the system of election and popular opinion automatically limit your options. Being vehemently opposed to a subset of policies does not mean supporting that candidate is automatically the wrong political play.
If you're truly a bright-eyed idealist and can't stomach the political sausage making then you'll have to find another approach. If you want to use the system in place (for example, USA's FPTP and electoral college) then you have to put in a ton of work and political maneuvering. If you don't believe in the reformist approach then you have no skin in the game and shouldn't be complaining.
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.
Deeply unserious person
If you can't fill in: "don't support X or Y but ___" then why make a comment tearing down any particular candidate? You're clearly not interested in what's possible in our current, real-life political environment.
If you're positive it's impossible to get anyone decent elected then go set something on fire or some shit. I'm not judging any radical position but we're talking about ballot boxes here.