I never said it was because of tribal mentality, though humans are social animals and have an innate drive to form into collectives comprised of individuals that share similar physical characteristics and/or psychological patterns with one another. Humans are naturally biased and prejudiced which goes hand-in-hand with tribalism. Republicans and Democrats are just two (very large) groups of humans that have similar viewpoints, with each one rejecting and opposing some (or most) of the viewpoints of its counterpart. Competition comes in the form of both parties vying for control of various sectors of the US Government.