front-runner vs contender

front-runner and contender both are nouns.

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

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

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

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