Which, for a hunter-gather, is a sensible way to do things. The people in your "monkeysphere" are the ones you depend on to survive. So do things their way, stick with them, and chant the chants they're chanting.
It doesn't seem like a sensible way to run an industrialized nation-state . . . but we're healthier, longer-living, and having more surviving offspring than our ancestors so we must be doing something right.
There are, of course, undecided voters. They're people whose identity isn't connected to one of the political factions and haven't been drawn into the debate. So they're going to go with whichever candidate manages to get to their group first. Then you have the "preference cascade" as a few members of the group take a stand and the rest align with them. Sometimes a loosely-committed group will have a few influential members decide to switch to the other side and trigger a cascade to take the whole group with them. Hence the frantic efforts by the presidential campaigns to identify key ("swing") demographics and carpet-bomb their members with propaganda.
But that's a small fraction of the population. So why are Facebook and other social media (such as the Livejournal of 2004) wallpapered with political memery? Well, that's not trying to persuade anyone on the other side, or even the uncommitteds. That's internal propaganda. People trying to convince their own tribe "I'm a good member" or "I should be one of the elders of the tribe" or "Trust me to fight against that evil other tribe!" I suspect some of the most frantic efforts come from people worried about being expelled from their chosen group for some heresy or just so low-status they'll do anything to cement their group membership.
I'm cranky (and autistic) enough to not be a true member of any tribe but I still feel the reflexes. I picked my presidential vote and am emotionally part of that "team", enough to be annoyed by attacks on Romney & Ryan. So I'm avoiding Joss Whedon's anti-Romney video until the election's well past. Then, like decade-old Doonesbury cartoons, I can enjoy it just for the humor without needing to care about any impact it has. But since I'm not trying to impress any fellow tribe-members I'm not posting any elaborate rationales for why my guy's so good you must vote for him or that the other one is so evil you can't possibly vote for him and remain my friend.
I suspect the vitriol of the arguments is worsened by the mechanics of our system. When only two parties have a chance at winning nasty behavior driving someone out of the opposition party is a net gain. If we had a system that allowed more than one party to hold real power the activists would be forced to play nice(r) or see a third party benefit at the expense of themselves and their targets.
*For anyone interested in the actual science behind this I strongly recommend Haidt's The Righteous Mind. It's a fascinating book.