write-in candidate vs nominee

write-in candidate and nominee both are nouns.

Word	Noun	Adjective	Verb	Adverb
write-in candidate	Yes	No	No	No
nominee	Yes	No	No	No

As nouns, nominee is a hypernym of write-in candidate; that is, nominee is a word with a broader meaning than write-in candidate:

write-in candidate: a candidate for public office whose name does not appear on the ballot and so must be written on the ballot by the voters
nominee: a politician who is running for public office

Other hypernyms of write-in candidate include campaigner, candidate.

write-in candidate (noun)	nominee (noun)
a candidate for public office whose name does not appear on the ballot and so must be written on the ballot by the voters	a politician who is running for public office