sue, I can't tell, without more context. If it's something like, "His access to a lot of money was a driving force in his entrance into politics", then your idea of "key" would apply: easy money was a key force, we might say. If it was something like, "Good grades are a driving force in the entrance into a top university", then it would be seen as more of a prerequisite.
I realize there is not much actual differentiation here. The best explanation I can offer for my two examples is that, in the first instance, we are left thinking that perhaps if the candidate had NOT had a lot of money, he might not have chosen to go into politics.