WordNet records no synonym and no antonym for coattails effect. It is the only member of its synset, 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.
(politics) the consequence of one popular candidate in an election drawing votes for other members of the same political party
“he counted on the coattails effect to win him the election”
Synonyms, antonyms and the broader and narrower relations for coattails effect come from Princeton University’s WordNet. Full sources and licences.