I don't agree that all those incumbents merit primary challenges, or that successful ones will necessarily result in upgrades.

The author is continuing what he did when he co-wrote the "Primaries For Progress", later "Primary School" Substack newsletter: recommending ways to shift the Democratic Party to the left in primaries both of incumbents and in open seats, but limiting it to districts or offices where there's little or no chance of doing so backfiring by potentially flipping the races to the GOP.