Is a principled politician one who only represents the people that voted for him? Does holding to your values mean that there is no room to care for your entire constituency? When the people vote for representation, the losing side loses its right to be heard. Tyranny of the majority can only be stopped if the people elect fair-minded politicians — those that see the virtues of working for all citizens, even the ones that did not vote for them.
They cannot have a preconceived notion of who and how they are to lead; they are no longer afforded that luxury. Just as soldiers give up their freedom in order to serve and relinquish their personal values as a sacrifice to protect the greater good, so must legislators.