declare has 3 distinct synonyms spread across 2 of its 8 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.
announce publicly or officially
“The President declared war”
state firmly
“He declared that he was innocent”
authorize payments of
“declare dividends”
designate (a trump suit or no-trump) with the final bid of a hand
make a declaration (of dutiable goods) to a customs official
“Do you have anything to declare?”
proclaim one's support, sympathy, or opinion for or against
“His wife declared at once for moving to the West Coast”
Synonyms, antonyms and the broader and narrower relations for declare come from Princeton University’s WordNet. Full sources and licences.