Full Idea
The citizen's vote is not a thing in which he has an option; it has no more to do with his personal wishes than the verdict of a juryman. ...he is bound to give it according to his best and most conscientious opinion of the public good.
Gist of Idea
Voting is a strict duty, like jury service, and must only be aimed at the public good
Source
John Stuart Mill (Representative Government [1861], p.299), quoted by Jonathan Wolff - An Introduction to Political Philosophy (Rev) 3 'Representative'
Book Reference
Wolff,Jonathan: 'An Introduction to Political Philosophy (Rev)' [OUP 2006], p.97
A Reaction
Mill was also concerned that voters might pursue 'class interest' (which they currently do, big time).