res publica vs political unit

res publica and political unit both are nouns.

Word	Noun	Adjective	Verb	Adverb
res publica	Yes	No	No	No
political unit	Yes	No	No	No

As nouns, political unit is a hypernym of res publica; that is, political unit is a word with a broader meaning than res publica:

res publica: a politically organized body of people under a single government
political unit: a unit with political responsibilities

Other hypernyms of res publica include political entity.

res publica (noun)	political unit (noun)
a politically organized body of people under a single government	a unit with political responsibilities