Why not simply promising to the citizens? It's them who put him there, so it's to them that he have to be accountable.

>To whom are they making the promising to?
>Maybe some believe that they answer to a higher authority than the Constitution if they break their promise to do their best.
>
>I think It's all matter of trying to be accountable to someone higher than ourselves.
>I bet way back when it all started, swearing on the Bible had more significant meaning, like you're promisng God you'll do your best.