I am not so sure Michelle Bachmann cannot defeat Obama. The main obstacle is for her to get the nomination.
But if she does (a big IF, I admit), she has personal qualities that can make the difference. Another obstacle is that she will face hostile media that will try to ridicule her faith. There is also the issue of her being a woman, which may deter some voters who might think she is not fit to be the commander in chief for example. On another hand, she could turn the fact that she would be the first woman to be elected President into an advantage.
True, it is a long way, but it is not impossible.