Starting off with your credibility cut in half is a serious handicap, but that's just the first step in our calculation of the credibility index. If the initial probability that a person's position on a single factual issue is responsive to the evidence is at most 0.5, then the odds multiply as the issues do. That is, if your position on whether it will rain agrees with that of most people your side of the political spectrum (or for that matter with your friends, or your fellow shoppers at Whole Foods, or people who have the same eye color as you do), and so does your belief about Benghazi, then provisionally we should infer that the chances that your belief about the issues together is sensitive to the evidence are at most 0.25. Add, for example, agreement on the effect of fracking and you're at 0.125. At the conjunction of 10 shared factual beliefs, the probability that the conjunction of the beliefs is sensitive to evidence is about 0.0009. If we are assessing a person’s credibility by whether the evidence has anything to do with how that person forms beliefs, we very quickly get to the point at which that is vanishingly unlikely.
Now all of this proof happens in a world of stipulations: unanimous groups approaching claims where the evidence is perfectly split. Made-up worlds are the only places you can mathematically “prove” something like this, but the phenomenon of real people with extremely low credibility indices is all too real. You are perhaps thinking that your own side has the preponderance of evidence in each case. Of course you are, if you have a side—which is a good reason not to. But assign almost whatever probabilities you like, and after a run of cases of agreement on factual matters with people on your side politically and no disagreements, you've got essentially no credibility.
Finding the truth is not our only goal. Perhaps our capacity for belief is given to us by evolution in order to help us achieve unity and solidarity with one another, so as to be able to act in concert. We often believe for social reasons, to express our membership in a family, a community, a nation, or a movement. Political partisans and others who are very susceptible to peer pressure want to believe the truth, sort of, but not as much as they want to belong, and perhaps they would not, on reflection, change that. But certainly, no one ought to believe anything such a person says on the grounds that they said it; if that is how they decide what to believe, they've repudiated any attempt to say what's true. We often know precisely what a political partisan will say about some controversial factual matter before they open their mouths and before the evidence comes in. It is a mistake to regard someone like that as a credible source of information.
In trying sincerely to find the truth on a factual question, a good first move in a case where one has not formed an opinion or on which the evidence is equivocal is to critically examine and provisionally regard as irrational an opinion that is a consensus among people like you. The most credible people—the people who are responsive to evidence—often disagree with the consensus of people around them. I think we sense this as they speak, and on the other side, we often mistrust politicians or spokespeople for interest groups. It's not that they are lying in some particular case, exactly; it's that they long ago lost any commitment to reality.
When we live in rival unanimous systems of facts, we generate rival unrealities, dueling hallucinations. Perhaps that's how we ought to think of Red and Blue America: not as geographical or ideological regions, but as rival fictional universes, as though there's a war between Middle Earth and Narnia. People create a consensual illusion and confirm it to one another. This cannot in the long run be anything but a continual source of ever-more-unanimous error within groups and ever-more-extreme polarization between them.
If it is impossible to achieve social unity without a consensus of factual belief, then we are in a position of having to choose between unity and truth. But of course various forms of unity do not presuppose unanimity; one can love someone with whom one disagrees politically, for example. We should encourage configurations of people, from political movements to middle-school cliques to all of us together, to be open to heretics, dissenters, and eccentrics. We want groups that do not treat dissidents as dolts or monsters, or even as obviously wrong. There could even be groups—there have been groups—in which dissenters are valued rather than dismissed, ostracized, or executed. That's a very good idea, because dissenters are likelier to be right.