Despite sincere belief behind those opinions, common sense logic suggests that both sides cannot be mostly right all of the time or maybe even most of the time. Perceptions of reality and policy preferences of the left and right are mutually incompatible. In most cases, both sides cannot be mostly correct at the same time, but it is possible that both sides can be more wrong than right for some issues, i.e., both can be mostly wrong.
For the most part, these consistent differences of perception and opinion stem from the biology of human cognition or how humans perceive reality and apply logic to it. The phenomenon is called confirmation bias. Confirmation bias is very powerful and it operates mostly or completely unconsciously. Because this is a matter of innate human biology, the bias applies to politics, politicians, partisans and special interest advocates. Unfortunately, in politics biases can be and are routinely manipulated.
It is clear that human cognitive biases, including confirmation bias, constitute a severe limit on knowledge. The question is whether that can be changed. The clear answer is yes, if one has (i) the moral courage to acknowledge these things in themselves and (ii) the work ethic to try to deal with it. It literally requires effort to apply logic and moral courage to face unspun fact and unbiased logic because the results often undermine a person's ideology and values.
The easy thing to do is to let biases distort fact and logic into something psychologically acceptable. The moral courage to do this arguably is in very short supply. On top of that, the two-party system relentlessly applies spin to obscure the situation. The obvious point of relentless spin is simple: It helps keep average people distracted and deceived about how poorly both parties, their politicians and their pay-to-play system of politics has failed and betrayed the American public and the public interest. Instead of beating up on both parties, their politicians and corruption, average Americans just keep beating up on each other, “knowing” that they are 100% right and the opposition is 100% wrong.
1. The human understanding when it has once adopted an opinion ... draws all things else to support and agree with it. And though there be a greater number and weight of instances to be found on the other side, yet these it either neglects and despises ... in order that by this great and pernicious predetermination the authority of its former conclusions may remain inviolate. Francis Bacon, Novum Organum, 1620.
2. Spin in politics, an ever-present curse, is inviolate, constitutionally protected free speech. Spin is routinely used by the two-party system to enhance bias-based self-deception. Players in the two-party system are acutely aware of the fallibility of human cognition and they ruthlessly exploit that. It facilitates players quietly getting what they want while deflecting public attention into the ancient, endless, unresolvable disputes that polarizes average Americans on the left and right.