Ideologue
Etymology
From French "idéologue," first used for the circle of thinkers around Destutt de Tracy, the original "science of ideas" scholars. The term picked up its dismissive tone the same way "ideology" did — through Napoleon’s scorn — and now usually implies inflexible, doctrine-first thinking rather than mere expertise in ideas.
noun · /EYE-dee-uh-log/
A rigid adherent of an ideology, who applies its doctrine uncompromisingly.
What it names
A pointed label for the person who judges reality by the theory rather than adjusting the theory to reality.
Example
The committee needed a pragmatist, not an ideologue who would sink any compromise on principle.
Related idea
A partisan simply takes a side; an ideologue is bound to the doctrine itself, judging events by the theory rather than the reverse.