It may seem that to be a good US Mormon you have to be a Republican. Fact is, the LDS church does not support any political party or candidates. The reason most LDS members tend to vote republican is because of the church's stand on traditional family values. Personally, moral issues regarding marriage and abortion take precident over economic and foreign policy. That said, I consider myself Independent or Conservative rather than Republican. Despite the Democratic Party platform, some of the most faithful and stalwart members of the LDS faith consider themselves Democrat (i.e. Senator Harry Reid D-Nevada). If the LDS church were to endorse one political party, the other party would loose an important moderating influence.
It's rather ironic that the US is battling religious extremism overseas while a fierce political battle involving political extremism is being waged at home. Both the far-left(secular progressives) and far-right (neo-cons) are radicalizing and divisive forces within their respective parties. I think it's important that there is a strong moderate LDS voice within both parties.