"But when party and officeholder differ as to how the national interest is to be served, we must place first the responsibility we owe not to our party or even to our constituents, but to our individual consciences. But it is a little easier to dismiss one’s obligations to local interests and party ties than to face squarely the problem of one’s responsibility to the will of his constituents. A Senator who avoids this responsibility would appear to be accountable to no one, and the basic safeguards of our democratic system would thus have vanished. He is no longer representative in the true sense, he has violated public trust, he has betrayed the confidence demonstrated by those who voted for him to carry out their views."