lobby has 14 distinct synonyms spread across 3 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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 large entrance or reception room or area
the people who support some common cause or business or principle or sectional interest
an interest group that tries to influence legislators or bureaucrats to act in their favor, typically through lobbying
detain in conversation by or as if by holding on to the outer garments of; as for political or economic favors
Synonyms, antonyms and the broader and narrower relations for lobby come from Princeton University’s WordNet. Full sources and licences.