This question came up during a discussion with some friends after I placed a bet on a certain candidate (let's call him T) winning an upcoming presidential election in a Western country, call it USA. It was pointed out that I may have (inadvertently and infinitesimally) helped T: Due to the dynamics of a betting market the next person placing a similar bet will be offered (very slightly) worse odds, which is indistinguishable from the market believing that the chances of T winning just went (very slightly) up. In a way, this is very similar to stating that I would vote for T when asked in a poll.
The question now arises: Would it be legal for T to place a massive bet for himself (thus skewing the betting market)? Would it make a difference if it was known what his motives were (i.e., skewing the marked against just being convinced of his victory and wanting to make a profit)? Could he bet he would lose the election? Could T suggest to his voters they should place such bets?
(It seems very analogous to bribing people to vote for T in polls, so intuition tells me no. At the same time, why would they be forbidden from placing a bet anyone else is allowed to place?)
Edit to clarify: The question originally applies to online betting, e.g., on betfair. I would be most interested, however, in knowing if there exists a betting market where a presidential candidate could place such a bet.