left-of-center vs leftist

left-of-center is not a noun while leftist is a noun.

Word	Noun	Adjective	Verb	Adverb
left-of-center	No	Yes	No	No
leftist	Yes	Yes	No	No

As adjectives, left-of-center and leftist are synonyms defined as:

left-of-center and leftist: believing in or supporting tenets of the political left

Other synonyms of left-of-center include left-wing.

left-of-center (noun)	leftist (noun)
	a person who belongs to the political left

left-of-center (adjective)	leftist (adjective)
believing in or supporting tenets of the political left	believing in or supporting tenets of the political left