Republican vs politician

Republican and politician both are nouns.

Word	Noun	Adjective	Verb	Adverb
Republican	Yes	No	No	No
politician	Yes	No	No	No

As nouns, politician is a hypernym of Republican; that is, politician is a word with a broader meaning than Republican:

Other hypernyms of Republican include pol, political leader, politico.

Republican (noun)	politician (noun)
a tributary of the Kansas River that flows from eastern Colorado eastward through Nebraska and Kansas	a person active in party politics
a member of the Republican Party	a leader engaged in civil administration
	a schemer who tries to gain advantage in an organization in sly or underhanded ways