Fast forward through some pretty aggressive and technologically advanced gerrymandering, and something like 90% of congressional seats are pretty well insulated from drastic party changes election over election. The primary becomes the only election that matters for most districts, which pushes both parties to the extremes. Because the primary is the only one that matters, there is little to no incentive to rally around your chosen candidate - you just fight that much harder for your favorite one next time.
Solidifying party control in most districts has somewhat paradoxically weakened party control over the process while simultaneously pushing elected candidates to the extremes.
This is what I really like about Ireland's elections - they are so effective in weeding out extremists that the two dominant political forces are almost indistinguishable. It's, perhaps, too efficient in that, but it also seems to be extremely safe and long-term stable.