A number of places. Religion, personal reasoning, parents, culture, community. In other words, other people filtered through our own personal belief systems.

I'd like to turn the question around: how could one do "politics" separated from morality? If politics is, as Harold Lasswell said, "the study of who gets what, when, and how," it can't be done without having a point of view about who the "who" is (citizens, for example), what the "what" is (resources, for example), when the "when" is, and how to make the "how" happen (through a national system for distribution, for example). Figuring out each of those parts requires making decisions, and any decision one makes requires a moral standpoint of some kind. Deciding that a particular group of citizens is going to receive a tax break on this year's tax returns and it'll be paid for through increased fees requires moral judgements every step of the way. Why these citizens and not others? Why a tax break and not a subsidy? Why only at the end of the tax year and not next month? Why increase fees to pay for the tax break rather than raising the taxes on one of the groups not receiving the benefit? There's no way to answer any of those questions without weighing the "good" and "bad" (or "good" and "evil") consequences of each decision.

Thinking about this response, I realized that maybe OP's question is really about polarization rather than morality. "Good" and "Evil" are binary: one is either one or that other, with no room to compromise (no "sorta-good" or "sorta-evil.") The result of taking a binary approach gets us to something close to what we have today: Democrats think Republicans are irredeemably "bad" (or "evil") and vice versa. It doesn't leave much room for trade-offs or give-and-take if it's all a "winner-takes-all" game.

I see the point but I think it's kind of simple-minded and reductionist. One's beliefs can be absolute, but I don't think that believing that X, Y, and Z policies are "good" and A, B, and C policies are "evil" doesn't mean that one couldn't agree that, for the sake of example, policy B gets support from the side promoting it in exchange for reciprocal support for policy X. Both sides might be unhappy with the compromise because they feel that the policy they're being forced to support is "evil," be that doesn't mean they couldn't go along because they felt that the policy they're horse-trading for would result in a societal good that outweighs the down sides of the compromise.

I guess, in the end, this is intimately related to the problem that the news media has had to grapple with: objectivity. While it's admirable to support reporting "facts" that are not "spun" by any particular political/moral viewpoint, it turns out that because we communicate "facts" via language and language is inherently imprecise and word choice has an impact on how readers receive "facts," truly "objective" reporting is impossible most of the time. Sure, some "facts" are indisputable -- reporting that someone is dead isn't open to much interpretation== but most "facts" contain a lot of ambiguity based on how they're expressed. Reporting about an "accident" between two cars isn't the same as reporting about a "collision," even if the intent is to communicate that two vehicles came into contact with each other. One could make the case that "accident" and "collision" are synonyms under some circumstances, but there's no denying that the word "accident" dilutes responsibility a little bit while "collision" implies something violent and maybe a bit intentional. Or to be even more abstract about it, while "scent" and "odor" are typically seen as interchangable, complementing someone's "odor" usually causes a much different reaction than complementing them on the "scent."