debate vs niggle

debate is a noun but niggle is not a noun.

Word	Noun	Adjective	Verb	Adverb
debate	Yes	No	Yes	No
niggle	No	No	Yes	No

As verbs, niggle is a hyponym of debate; that is, niggle is a word with a more specific, narrower meaning than debate:

debate (noun)	niggle (noun)
the formal presentation of a stated proposition and the opposition to it (usually followed by a vote)
a discussion in which reasons are advanced for and against some proposition or proposal

debate (verb)	niggle (verb)
argue with one another	argue over petty things
discuss the pros and cons of an issue	worry unnecessarily or excessively
have an argument about something
think about carefully; weigh