struggle vs tug-of-war

struggle and tug-of-war both are nouns.

Word	Noun	Adjective	Verb	Adverb
struggle	Yes	No	Yes	No
tug-of-war	Yes	No	No	No

As nouns, tug-of-war is a hyponym of struggle; that is, tug-of-war is a word with a more specific, narrower meaning than struggle:

struggle (noun)	tug-of-war (noun)
strenuous effort	any hard struggle between equally matched groups
an energetic attempt to achieve something	a contest in which teams pull of opposite ends of a rope; the team dragged across a central line loses
an open clash between two opposing groups (or individuals)

struggle (verb)	tug-of-war (verb)
be engaged in a fight; carry on a fight
to exert strenuous effort against opposition
climb awkwardly, as if by scrambling
make a strenuous or labored effort