Ideology may set rough attractors and no-go areas, but it's naive to think that our current battle lines have been drawn by individuals independently pondering their own positions.
At a deep level, our experience of reality has become wholly moderated by mass media. Reds and Blues are watching different channels, and thereby experiencing different realities. It's as simple as that.
Nonconformance to a media narrative is punished by all, in a distributed fashion. If you express an independent point in a Blue flavor, you will be attacked by both the ever-present Reds as well fellow Blues for breaking rank (and vice-versa, obviously).