This is quick and easy, a syllogism for voting:
Each moral decision ought to be based on the natural law.
Each moral decision ought to be unaffected by the trends of how other people make the same moral decision.
Voting is a moral act.
Therefore, the act of voting ought to be based on the natural law and be unaffected by the trends of how other people vote.
So, if you actually care that your act of voting be good rather than sinful, then you must not weigh probability of candidate victory in your choice about for whom to cast your ballot. Simply: Using “electability” to help determine your vote is evil.