front-runner vs rival

front-runner and rival both are nouns.

Word	Noun	Adjective	Verb	Adverb
front-runner	Yes	No	No	No
rival	Yes	Yes	Yes	No

As nouns, rival is a hypernym of front-runner; that is, rival is a word with a broader meaning than front-runner:

Other hypernyms of front-runner include challenger, competition, competitor, contender.

front-runner (noun)	rival (noun)
a competitor thought likely to win	the contestant you hope to defeat

front-runner (adjective)	rival (adjective)
	as a rival in a competition

front-runner (verb)	rival (verb)
	be the rival of, be in competition with
	be equal to in quality or ability