[quote="Kal El"]Possible. I wish we had more information. It could also have been someone high up in the administration and close to Dasaratha and thus aware of such sensitive issues. Rama could have learned about this from such a person while he was still in the palace and digesting the news about the exile and abdication. [/quote]
In that case, wouldn't and shouldn't Sumantra and Vashistha have been aware of this pledge? I have a theory here that just as 'rajdharm' forced Rama to remove Sita as queen on the basis of widespread controversy, similarly, Vashistha and Sumantra knew about the pledge, but both determined (not opined) as per shastras that under rajdharm, which would trump anything as far as succession rules go, and so the normal rules of succession would trump any vow that the king made to anybody. Unless the kingdom itself would be threatened (like it would have been had Harishchandra not turned it over to Vishy meister), there was no question of the ruler giving up the throne just to satisfy his punya-kamna or any other spiritual goals.