I've always been struck by the strength and depth of binary choices. Frankly, discussion forums on the internet, with their world-shrinking, illusion of privacy and instant gratification make this all... easy.
And oft repeated quote is "Reasonable people can disagree". I'd say, ironically, that reason has very little to do with it.
David Van Biema, elucidating noted anti-religion author, Sam Harris', study reported in the Annals of Neurology, wrote in Time Magazine
Statements like "2 + 2 = 5" and "Torture is good" caused an area called the anterior insula to light up. True statements like "2 + 2 = 4" activated the ventromedial prefrontal cortex. The ventromedial is thought to play a role in judgment, memory, fear and, according to one study, soft-drink preferences. The anterior insula helps process fear, disgust and reactions to bad smells.
This is not the only study to have suggested that disbelief and moral outrage may be processed in the area of the brain that makes us go "Blechh." Sam Bowles, professor of human behavior at the Santa Fe Institute, describes research in which an unfair business deal produced a response in the same region. How did disgust get involved in the belief-and-disbelief business? Some think it started as a fairly straightforward adaptation to enable a suspicious taste, smell or appearance--like that of vermin--to trigger the impulse to eliminate the source. We may have then generalized that reaction to ideas. "When someone says something you disbelieve," Harris says, "it has a kind of emotional tone. Rejecting someone's statement as illogical or incompatible feels like something."
I think that most reasonably intelligent people have a basic understanding that people hold beliefs tenaciously, and that people argue about the dumbest stuff... but I don't know that its common knowledge that its a primordial imperative.
I should also note that, logically this "disgust reaction" is much more likely to occur, and (in personal experience in stronger measure) in decisions with only two choices... "Sony or Nintendo," "True or False," "God or No God," "Good or Evil" and "Hillary or Obama."
The binary nature of the contest forces those who are partisan, to not only choose one candidate as right, inevitably mean that one is wrong... a monster in terms of our primal brain, because we feel disgust about a clearly wrong choice.
This is precisely the reason that Clintonistas can ignore "math" or "reality". The math is still "uncertain" even if the situation is mathematically very solid. The strongest partisan situations will cause people to ignore even blatantly contradictory facts, too.
Clinton supporters... and Obama supporters have not only developed a desire to see their "guy" become elected president, but the mind forces us to not only think of the opposite in negative terms, but to feel "icky" about the alternative... so the next time you are having a partisan moment, you might want to ease up on the returning nasty rhetoric... not because of "party unity", but (honestly) their brain made them do it.
(Update) haremoor raised a point regarding fear being a motivator, and I'd agree that has an effect in politics.
The "disgust of the other, wrong choice" and the "fear of the alternative" situations has pretty defined requisites in politics: The situation here... choosing your candidate, and then having an adverse feeling to the opposition, can be found in voters with a high initial positive reaction to that specific candidate. The alternate candidate will, almost without fail, later see a rise in their negative ratings in polling.
The fear reaction is different. I'd say you'd find that in voters with a higher initial negative for the attacked candidate... the other candidate will then usually see a corresponding rise in their own numbers, because you've associated a negative connotation with that one candidate... they feel wrong and the brain seeks its needed alternative.
When both candidates are viewed negatively, the brain seeks the only alternative... not voting at all.