If Claire McCaskill is to upset Governor Bob Holden in the Democratic gubernatorial primary, she will have to buck history. Incumbent governors rarely face primary challenges. It has happened only once before in Missouri history. James Spainhower fell short in his effort to defeat Joseph Teasdale in the 1980 Democratic gubernatorial primary. Nationwide, 17 incumbent governors have been defeated in their party’s primary since 1970. None since 1994. University of North Carolina professor Thad Beyle tracks gubernatorial politics. He says money favors the incumbent and scares most challengers away. Beyle does point out that in each of the four gubernatorial elections held last year, the political party in power lost. A couple of other notes on successful incumbent challenges: of the 17 who won their party’s primary, 12 went on to win the general election.