Philosophers traditionally draw a distinction between normative and factual claims. We might distinguish them by noting that different sorts of reasons count for or against assertions about facts and those about values. Whether it would be a good thing for it to rain on Saturday in Fresno, or whether it would help us make progress if it does, is irrelevant to the question of whether it actually will rain Saturday in Fresno: That depends on whether drops of water actually fall out of the sky.
In sincerely trying to find the truth about factual claims, it is often important to decide whom to believe. To do that, it’s important to figure out who is advocating something on the basis of evidence. The people most likely to be sensitive to evidence—and therefore most worth listening to—often disagree with the consensus of the people around them on factual matters. We ought provisionally to regard people who frequently act as dissidents, heretics, and pariahs in their own political group as being more committed to speaking the truth than people who usually or always agree with the consensus. A person who diverges from the consensus of the people with whom she agrees politically may have other problems of credibility. But she does not start off with this one.
In order to build an argument for this, let's conjure some imaginary situations. First, imagine that we are psychologists, and for whatever reason we are conducting a study of the belief systems of people with green eyes. It turns out that they all believe that cutting taxes increases revenue. We'd be stunned by the arbitrary unanimity. But now imagine further that it turns out that green-eyed folks unanimously share many other beliefs: that there is a highest prime number, for example, that there is extraterrestrial intelligent life, that it will rain next Saturday, and that there was no cover-up about Benghazi.
One thing we could certainly conclude is that whatever they might say about themselves, many green-eyed people are not basing those beliefs on the evidence. That is because the evidence in each case is split, and we'd expect disagreement among people with green eyes, or would expect the split among people with green eyes to be similar to the split in the population as a whole. Whether you have green eyes is completely irrelevant to rationally assessing the effects of tax cuts on revenue. Something is making the green-eyed people believe these things, we must suppose, but it is not having reasons. Maybe it's genetic, or maybe having green eyes is associated with some sort of neurological glitch.
Now imagine that all the leftists in Fresno think it will rain on Saturday and all the rightists think it will not. We'd find that as arbitrary as in the case of the green-eyed people and tax cuts. Whether you are on the left or right has no connection to having reliable information on whether it will rain next Saturday in Fresno.