At, on and in are the main prepositions in English indicating position. Generally speaking:
in is used to specify position inside smaller areas;
on is used to specify position on a line or continuum;
at is used to specify position in a larger place.
Use of an appropriate preposition depends mostly on how you think about it.
An example. I heard certain news on the Radio-BBC world Service and saw it on the TV-BBC world and then wanted to verify the same in the news paper. So I can say:
'I saw it on TV-BBC World, heard about it on the BBC World Service and then read about it in the Guardian Weekly.'
In the example cited, the position of some one in the Legslative Council Eelection which is small area is described.So the appropriate preposition is 'in'.
Student or Learner