Generically and historically, there there has been the fachs of political philosophy (a branch of philosophy) and of political science (a branch of the social sciences). Political scientists are those who have traditionally been thought to study politics using the scientific method (acknowledging that many post positivists/modernists have concluded that there is no such thing as a scientific "method.")
The terms "Theory," and "Theorist," however, have come in the past half century or so, to have a narrow sense, to be specialized terms referring to "Critical Theory," which "designates several generations of German philosophers and social theorists in the Western European Marxist tradition known as the Frankfurt School."
According to these theorists, a “critical” theory may be distinguished
from a “traditional” theory according to a specific practical purpose:
a theory is critical to the extent that it seeks human “emancipation
from slavery”, acts as a “liberating … influence”, and works “to
create a world which satisfies the needs and powers” of human beings
(Horkheimer 1972, 246). Because such theories aim to explain and
transform all the circumstances that enslave human beings, many
“critical theories” in the broader sense have been developed. They
have emerged in connection with the many social movements that
identify varied dimensions of the domination of human beings in modern
societies. In both the broad and the narrow senses, however, a
critical theory provides the descriptive and normative bases for
social inquiry aimed at decreasing domination and increasing freedom
in all their forms....While Critical Theory is often thought of narrowly as referring to the Frankfurt School that begins with Horkheimer and Adorno and stretches to Marcuse and Habermas, any philosophical approach with similar practical [and normative] aims could be called a “critical theory,” including feminism, critical race theory, and some forms of post-colonial criticism. (See https://plato.stanford.edu/entries/critical-theory/. To understand the distinctions I make here, you must have a basic understanding of Frankfurt School "Critical Theory," pretty much as described in this SEP article.)
These Marxist political theorists are to be distinguished from political scientists, in that they have a practical, normative, goal oriented "consciousness raising" project in mind when they "theorize," born of their fundamental dissatisfaction with the fact that "the correctness of any [scientific, impartial] explanation is independent of its desirable or undesirable political effects on a specific audience." (Id.)
That is, they do not seek to gather data with the aim of attaining "knowledge" about the world, or to articulate a rational basis for their theorizing, as most traditional social scientists would describe their project. Their constructivist aim, is edification, to change the world, to make it, by their "enlightened" lights, more "equitable. In short, to instantiate Marx's 11th thesis on Freubach, "Philosophers have hitherto only interpreted the world in various ways; the point is to change it."
And this, in a nutshell, is what distinguishes a political theorist, from a political philosopher on the one hand, and a political scientist on the other. The Theorist, in the contemporary post positivist sense, is a reformer/reformist, while the traditional philosopher and positivist scientist are [or, according to the theorist, naively believe themselves to be -- given exuberant interpretations of the "myth of the given" and the "theory ladeness" of observation, etc.] merely curious inquirers seeking to explain, make sense of [political] phenomena/states of affairs.