WordNet records no synonym and no antonym for bandwagon. 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.
a popular trend that attracts growing support
“when they saw how things were going everybody jumped on the bandwagon”
a large ornate wagon for carrying a musical band
“the gaudy bandwagon led the circus parade”
Synonyms, antonyms and the broader and narrower relations for bandwagon come from Princeton University’s WordNet. Full sources and licences.