The noun leftist has one meaning:
Meaning #1: a person who belongs to the political left
Synonyms: collectivist, left-winger
The adjective leftist has one meaning:
Meaning #1: believing in or supporting tenets of the political left
Synonyms: left-of-center, left-wing
a person who belongs to the political left
Anyone belonging to "the left", could include anyone from John Kerry to Noam Chomsky.