across the country vs nationwide

across the country is not an adjective while nationwide is an adjective.

Word	Noun	Adjective	Verb	Adverb
across the country	No	No	No	Yes
nationwide	No	Yes	No	Yes

As adverbs, across the country and nationwide are synonyms defined as:

across the country and nationwide: extending throughout an entire nation

Other synonyms of across the country include across the nation, nationally.

across the country (adjective)	nationwide (adjective)
	occurring or extending throughout a country or nation

across the country (adverb)	nationwide (adverb)
extending throughout an entire nation	extending throughout an entire nation