Why Are Politics so Religious Like in the US?

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I'm from Sweden and politics was never really anything big in my life. And I have been going back and forth to Washington DC for a long time now (3-4 years actually!) moved here October 07, and got to experience both the presidential campaign, the election and inauguration. What I don't understand is how people can be so one or the other about politics here. My fiancées mom is a hard core republican who honestly usually votes against her own interest. She only voted republican because that's what she's born into, just like religion. Me myself is neither republican or democrat, I don't know if there's a word for that. Why? Because I don't want to affiliate with anything that people get so upset about. I work in retail and during the campaign I was saying that I liked Obama better than McCain (mostly Palin threw me off) and that affected buyers greatly. If they disagreed they would buy less. Get gruff and sometimes rude at the edge of mean. Most people I know here don't even realise it themselves. Anyone else feel the same way? Or are now mad at me for being too aggressive? ._.