I wouldn't call it his sole mistake. Akin had a lot of quotes for McCaskill to call upon - against Social Security, against Medicare, against the minimum wage. As it happened he managed to do himself in before McCaskill had to haul the other quotes out. Akin was a disaster waiting to happen from the moment he got the nomination. That's why McCaskill was so glad to run against him.
How would those other quotes be “mistakes ?” He’s a Conservative in his votes, not just a talker who turns around and votes for the liberal establishment. It was one thing and one thing only that did in Akin (and same thing for Mourdock in Indiana), and he should’ve been sophisticated enough to know to avoid the topic or state that he is pro-life and keep reiterating the point. If Akin was a “disaster waiting to happen”, he certainly would not have enjoyed a long and successful career in the House. McCaskill took an enormous risk in trying to get Akin to be her opponent, and it just as easily could’ve blown up in her face. The same thing happened a decade ago when backing a certain Kenyan with a name that sounds like terrorists for an Illinois Senate seat (with the thinking that an initially well-regarded Chicago Republican would take him down), and we see how that turned out.