Copied from comments made before this question was migrated from ELU to ELL...
(FumbleFingers Reinstate Monica)
[nor will they accept] even much tactical vagueness = progressives will only accept relatively small amounts of vagueness (they won't accept very much).
It might look a bit odd to a non-native speaker reading it (and incorrectly parsing it), but that phrasing sounds perfectly natural to native speakers.
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(Chappo Says SE Dudded Monica)
While much normally means "a lot (of)", the combination of even with no longer gives it a negative sense, similar to "I haven't got much money" (meaning "I have very little money").
Sanders has decided that progressives will accept very little tactical vagueness from now on. A native speaker would understand the usage, even if it's somewhat idiomatic (and therefore not so easy to explain)