The nature of politics is difficult, to serve the best interest of the people and the nation. These may not always be in alignment, especially at a national level. The elected official takes an oath of office to represent his constituents to the best of his or her ability while at the same time they are working with others to legislate sometimes momentous national issues, including:
There will be times when a legislator must make decisions that will necessarily be at odds with the best interest of local constituents while being in the best interests of the nation as a whole. Which is the better action? Which is the more ethical action?
To serve the greater good is always the more ethical action.
While serving the greater good may be against self-interest (re-election) public service requires sacrifice of self interest to larger national interests.
I have intentionally left out of this answer the issue of private monies and lobby's and assumed a politician is honest in his / her desire to serve. That the only ethical dilemma. This is a much larger issue and one that needs a much more in depth answer, where larger amounts of money is involved goes beyond the issue of ethics in my opinion.