Term referring to a politician who does nothing but appease the masses. The white-horse aspect refers to the fact that such a politician will "ride in and rescue all the people from their sorrows". The man-on-a-white-horse typically labels himself as a champion of the common man. Those who don't support the man-on-a-white-horse label the politician as the opiate of the masses.
Also connotates a politician who wins or attempts to win an election based solely upon the contrast between himself and his opponent. Usually this occurs when a nation is in some sort of turmoil or when opposing politicians are corrupt or overbearing upon the people.
In essence, this is a negative term used by historians or other political analysts to denote a reform candidate who would be unable to win if it weren't for the opposition. Also, it suggests that the man-on-a-white-horse's platform is not well thought out but only there to denote him as "different", as well as to get votes.
Example: Some consider Vladimir Putin to be a white horse candidate.