C. S. Lewis, back in the 1940's, noticed a particular tactic of argument that was so pervasive that he felt he had to give it a name. He called it Bulverism(it is also called the genetic fallacy). Lewis defines Bulverism as "instead of trying to prove your opponent wrong, you assume he is wrong, and give an explanation of how he got that way." Bulverism essentially amounts to dismissing an opponent's arguments because you can imagine some irrational motivation for them believing as they do. As Lewis put it, "Until Bulverism is crushed, reason can play no effective part in human affairs". Seems to me that the latter phrase comes pretty close to describing present-day politics
Don't imagine, though, that Bulverism is confined to one political party. Conservatives bulverise the liberals about as much as liberals bulverise conservatives (though I'll admit that some on the left seem to have turned Bulverism into something of an art form. E.g. 'Homophobia' is, essentially, a one-word Bulverism). That's essentially the problem - everyone ends up pointing fingers at the alleged motivations of their opposition instead of actually arguing on the merits.
It can be amusing to see those claiming to be speaking as scientists using Bulverism (and they do), as Bulverism essentially is hypothesizing a motivation for your opponents reasonings, assuming without examination that that hypothesis is true, and proceeding to dismiss your opponent's reasoning on that basis. This is about as far from the scientific method as you can get.
Bulverism is attractive because it gets around the tedious, difficult process of having to understand your opponents arguments, figure out where they're wrong, and proceed to demonstrate those errors logically (followed by your opponent doing the same to you). All that's required is a bit of imagination and the willingness to risk slandering(or libelling) your opponent.
As far as I can see, the pervasiveness of Bulverism has increased since the 1940's, to the point where it is almost taken for granted. The opposition isn't just wrong, they haven't merely made mistakes in reasoning, they must have gotten there due to some nefarious motivation. And once the imagined connection between 'wrong beliefs' and 'bad motivations' is assumed, disagreement becomes intensely personal. If being incorrect implies evil motivations, if mere disagreement with someone implies that you think their motivations are bad or wrong, people are going to resent disagreement. The end result is that, instead of arguing about "who's right?", we end up arguing about "Who's righteous?".
To me, the amusing thing about this (if such a state of affairs can be regarded as amusing) is that in my experience, the vast majority of people are actually extremely lousy at discerning other peoples motives. We regularly assume that other people's motives can always be reduced to one motive, when in fact we generally operate under the weight of multiple, often conflicting motives. We seem to ask "What would it take for me to do that/believe that?", and assume that that must be the other person's motivation, regardless of how different they are from us. We too easily assume our own ability to see into the hearts of others based on appearances, when in fact only God sees into our hearts. Not only is Bulverism intellectually lazy, it depends on an ability that only few come anywhere near having competency in. And this is what drives political discourse nowadays? You have to laugh, if only to keep from crying.