A separate, though just as divisive topic, is abortion rights. I've watched for 20+ years as each side screams across the aisle and yes they have different opinions but at the end of the day it has always seemed to me a battle over where to draw the line. Both sides support new legislation, and therefore new governmental reach, defining what a person can do with their body. They can't agree on when an unborn child is considered human and must be protected, but both sides want the government to tell us what we can or can't do.
Growing up in the 90s the parties were always pitched as big government vs small government, balanced budget vs spending for social causes, war hawks vs anti-war, etc. My opinion at this point is that we have two parties that are different in degree but not kind - both are pro-war, big government, money printing machines that have done a great job at keeping up a front of a two party system. This is why I say the left/right split seems lost here today, I don't see any divide in core principals driving the parties.
Granted, though, that for anyone who is a one or two issue voter there can be very different policies that would push one to either side.