The above (rational, civil discourse) CAN happen, people just CHOOSE not to engage in civil discourse. It is all a choice. Right now, the far left CHOOSES to view the right as a bunch of jackbooted, evil, racist Nazi fascists that want to kill grandma and throw the poor out into the street to starve. And the far right CHOOSES to view the left as Commie sympathizers intent on shredding the constitution, raising taxes to 100%, and imposing mandatory gay marriage on everyone.
People are CHOOSING to view the other side as cartoon caricatures, so their ears perk up when they hear anything remotely incendiary, and they CHOOSE to take the rhetoric the wrong way so they can raise a big stink about it and say, "See? That PROVES that the other side is the enemy! They're violent and hateful!"
It's all very childish, and it's what is driving sensible people out of the two main parties, in my opinion.