Zocalo Public Square hosted a forum on big and small political donations and their effects on campaigns. Here's a chunk of my take on the issue:
The CEO of Worldwide Acme Corporation and Joe Schmo from Ohio give to politicians for the same reason: They think their guy is going to get them something they want. What they want might be selfish (a tax break or a subsidy that benefits them directly) or it might be altruistic (more immigrant visas or transfers to the poor).
But more and more people think it's worthwhile to drop some dough in the pockets of politicos because office holders have more and more power every year. Government continues to grow in size and scope, which means legislators, presidents, and city councilmen have more money to give away, more contracts to dole out, and more tax advantages to confer. Rather than putting people and companies in a position where they believe they have to support (or buy) a public official to get what they want, I'd rather see a scenario where government is smaller and people have to figure out other ways to spend their money to change the world.
Go check out the rest of the forum for donation-induced panic, pleasure, and everything in between.