Intrade usually takes into account the fact that they're mutually exclusive, and doesn't require additional margin for additional candidates (I haven't seen this documented anywhere, but it seemed to behave that way in the last election).

Still, Intrade's margin rules aren't as good as those of the big commodity futures exchanges. Interest rate futures have asymmetrical risk, but that doesn't distort prices because little margin is required and they're liquid enough that you can cut your losses before prices change too much.