Ezra Klein reviews a couple of books on the subject of lobbying and money in politics. He comes to the conclusion (contra the authors):
that as big a problem as money is in politics—and make no mistake, it is a big problem, as the rise of the Super PACs shows all too clearly—it is not the only one, and it is probably not even the worst one.
His main piece of evidence is that while lobbying can (and does) influence legislation, it does so less for popular items than for smaller items. But he doesn’t address a number of issues. I think the most important issue is how campaign contributions effect who is elected and who is not. That can have a big influence on which laws get passed.