After all, due to McCaskill's unpopularity in Missouri, Akin just has to be perceived as a more or less acceptable alternative, he doesn't need to be loved by everyone.
Rape isn't something taken lightly over here. When your party is already involved in a War on Women, and you start trying to tell these women that they were not legitimately raped, you wouldn't be able to overcome that, especially against a female opponent. We've seen the GOP lose races because of this as recently as 2010 in Colorado.