Maybe you received a low grade, not because of the subject matter of your review (which I fail to see as being conservative or liberal), but because the review was poorly written.
Maybe, however as a straight A+ student of history with a near perfect score on my verbal SAT, plus the fact the paper was just fine and the professor’s “American History” class was 100% revisionism that FDR was a conservative, Lincoln a racist, etc. - means that you’re mistaken and the professor was a liberal jerk who on a daily basis couldn’t tolerate the though of anything not liberal.
I didn’t think such occurrences were met so skeptically here, professor.