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.
(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)