There is, of course, good reason to be truthful. I’m strongly inclined toward Kantian deontology in moral theory, so I’m inclined to think that the most important reasons are of a non-consequentialist variety. But I’m not going to discuss those reasons. Instead I’m going to discuss some less philosophical, less fundamental reasons for being truthful in politics in particular.
What makes democracy valuable? Why, that is, is democracy a good (perhaps even the best) type of government? Here’s a suggestion: because it is the type of government that is maximally respectful of the autonomy of its citizens. Recognizing the inherent right of self-determination of every (adult) person, democracies attempt to maximize the degree to which such self-determination can be realized. Of course given that there are many individuals in any democracy, it is not possible for each one to completely determine policy. The best we can do, it seems, is to give each individual an equal say in determining policy, or at least an equal say in electing those who determine policy.
So what exactly—or, at least, approximately—is involved in respecting someone’s autonomy in matters of this kind? Let’s think about an analogy. Doctors are supposed to respect the autonomy of their patients. This does not mean that they leave all of the technical details of treatment up to the patient. Rather, it means that they must leave certain types of decisions up to the patient. In a case, for example, in which a decision must be made between undertaking a more effective but more painful treatment and a less painful but less effective one.
It is a mistake to think that what is important in a case like this is the mere act of making a choice, and hence a mistake to think that one is merely obligated to allow individuals to make choices. The doctor does not fulfill her obligations merely by allowing the patient to make a decision. Consider, for example, a doctor who lies to her patient. Suppose, in fact, that she lies to him about the evidence, exaggerating the likelihood that he has disorder A rather than disorder B, and using various rhetorical tricks to influence him to choose treatment 1 rather than treatment 2, even though an objective assessment of the evidence indicates that her diagnosis is wrong and her preferred treatment is sub-optimal. The doctor’s obligation to respect her patient’s autonomy requires that she allow her patient to make an informed decision. And that, of course, means that she fails to discharge her duty if she distorts the evidence she presents to her patient.