“In few other professions are you required, each and every day, to weigh so many competing claims— between different sets of constituents, between the interests of your state and the interests of the nation, between party loyalty and your own sense of independence, between the value of service and obligations to your family. There is a constant danger, in the cacophony of voices, that a politician loses his moral bearings and finds himself entirely steered by the winds of public opinion.”