Sinquefield gives quarter of a million to Chris Koster

Attorney General Chris Koster, just one day after receiving a quarter of a million dollars from the Democratic Attorney Generals Association and $10,000 from former House Majority Leader Dick Gephardt, picked up another $250,000 from retired billionaire Rex Sinquefield.

I am trying to figure this out. If Rex Sinquefield is backing Chris Koster and Chris Koster's opponent is Ed Martin, how in the heck can you vote for anybody in this race?