front-runner vs challenger

front-runner and challenger both are nouns.

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

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

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

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