I strongly suspect price fixing is occurring in the betting on elections, at least on the intrade market. I base that on the fact that these markets have relatively thin trading volumes, and I occasionally see a huge block of over a thousand shares push the price up, or another price down. These are positions that are WAY larger than those commonly put up.
I have no problem with this "campaign financing", and welcome it with open arms. A mosquito with small positions can quickly fly in, and take a little from these players, and get quite a return on investment.
For instance, the intrade positions in 2006, on which party would take control of the house and senate were clearly out of whack. I would see these huge positions push up the republican odds to completely unreasonable levels. Taking the other sides of these trades would have been very lucrative.
However, it does bring their accuracy into question. Markets CAN be fixed, if the floats are small. It is not at all illegal, so I think we should assume its occurring. Most especially since every tin-horn journalist out there is constantly quoting them along with the polls.
One could say...well...everyone will see the inefficiency, and make it go away. Sure, that would be true over time. But with a budget of...say...a few hundred thousand, I could make any given such market go my way for a month or two. Take a look at the volumes, and calculate it for yourself.