WordNet records no synonym and no antonym for grass roots. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the essential foundation or source
“the problem was attacked at the grass roots”
the common people at a local level (as distinguished from the centers of political activity)
Synonyms, antonyms and the broader and narrower relations for grass roots come from Princeton University’s WordNet. Full sources and licences.