I think it is useful here to distinguish politics as a consequence of morality from politics as a agreed set of methods of public decision-making. With the first politics, or politics(A), yes, one has to present all facts as they are regardless of whether they favor one’s stance IF one is to believe there is a moral duty to be rational. In a world where humans all share that particular view on morality, there won’t be a need for the second kind of politics, or politics(B). Because, in that world, the set of methods for rational decision making suffice as the method for public decision making.
But what if some of us do not share that particular view? I, for example, could believe one’s utmost moral duty is to conserve all forms of life, and, regardless of whether I am rational, I would present a view biased towards the outcome favored by my moral code. In that case, my bias is not the result of lack of rationality but one of my morality.
I agree with Eliezer that politics(B) is not an ideal place for rationality but I think it was never meant as such. I think(meaning my opinion) the democratic political system is envisioned to be an arena not of rationality but of morality. As such, it shouldn’t really matter how an issue is presented. Rational arguments appeal to rational voters. It was not a flaw of the system that some voters are irrational and someone presents an irrational argument to appeal to them.