<If large campaign contributions don't have that great an influence in the outcome of elections, can there be any doubt that the money is intended to influence the decisions of elected officials after they're elected? Aren't campaign contributions from corporations bribes in every meaningful sense?

Without a doubt. And I don't think any reasonable person would disagree with you. And let's not forget campaign contributions from entities other than corporations, like unions and hundreds of PACs and special interest groups. Ayn Rand called it the Aristocracy of Pull. As I mentioned in the post above, if the motivation is not bribery, it is protection money. The latter may be more morally defensible on the part of the donor, but it is no less disturbing.