What’s the opposite side of the center?
If you look at politics as a spectrum of seven steps (far-left, left, center-left, center, center-right, right, far-right) and then consider US politics (relevant because you live there), then the opposite of what you perceive as center is left. That's because the republican party has leant to the right to begin with, covering interests of the right and far-right whereas the democrats cover everything from far-left to center-right on the spectrum. Recently, Trump has moved the republicans even more to the right, so the current split between left and right in the USA means that plain right is essentially the current center in American perception. The opposite of that is plain left.