I've always wondered if someone might try this. From CQ Politics ...

An internal investigation by the popular online market Intrade has revealed that an investor has been attempting to artificially boost the prediction that Sen. John McCain will become president.

Over the past several weeks, the investor has pushed hundreds of thousands of dollars into one of Intrade's predictive markets for the presidential election, the company said, resulting in great financial losses through a strategy that belies any financial motive.