I suspect that, in some small way, all human reason is impaired. As long as a politician makes choices with which I agree, and his motives will not affect future decisions, why should I care?

Take environmentalism for example. What if Politician X wanted to pass legislation that would truly protect the environment because the Bible told him that we are to be good stewards of the Earth. Why should his motivation be considered if the end result is for the greater good?

Take Planned Parenthood for example. What if Politician X wanted to increase availability of contraceptives or safe, legal abortions... because they believed that it would reduce the number of babies born to minorities or the lower classes? How could you continue to support someone with evil motives even if you agree with contraceptives or safe, legal abortions

I would assume that a decision based upon flawed logic evinces a flawed method of decision. As such, the hypothetical politician is likely to make decisions with which I disagree; his faculties of deduction and reason cannot be trusted.