Fiduciary Duty: Simple Ethics Concept for Government Officials:

The English word "Fiduciary" comes from the Latin fides (faith) and fiducia (trust). (Wikipedia).

A fiduciary has engaged in a relationship with his/her client that includes a trust and obligation (duty) to act only in that client’s best interests – without gain or benefit to the fiduciary.

Under the US Governmental structure: We, the citizens of the USA, elect our government officials to act as fiduciaries on our behalf. This democratic election process is the ethical grounding for giving our leaders the power and authority to run our country. 

By virtue of being elected – and hence having our trust placed in him/her, he/she is obligated to use an ethic of duty and care on our behalf in all his/her official actions. Period.