In the New York Review of Books, Ezra Klein makes a convincing argument that the way lobbying has corrupted American democracy is not through money, or even indirectly via a "gift economy," as Harvard's Larry Lessig has argued, but rather because lobbyists provide legislators with a legislative subsidy.
If cash and lobbying bought votes, you'd expect a law like PPACA to have gotten some bipartisan approval, given that it was supported by major industry groups. Instead it was a party-line vote. Lobbying works not by swaying votes on the big-ticket issues, where politicians are more accountable to voters and their parties, but on the small-time issues that don't get public attention: Jack Abramoff's biggest job was getting tax breaks for Indian tribes, not exactly a page A1 issue. The way the legislative subsidy works is that on these host of smaller issues, lobbyists are a source of information, research and argument that simply isn't matched by the other side.
If someone walks up to you with a bag full of money and asks you to vote to make coal companies more profitable, that's not a very persuasive argument. Even if you take the money, you're going to feel dirty the next day. And most people don't like to feel dirty. But if one of your smartest, most persuasive friends, a friend you agree with on almost everything, is explaining to you that those environmentalist nuts are going too far again -- they're always doing that, aren't they? -- and they have sneakily tucked a provision into a bill that would make it more expensive for your constituents to buy electricity, that's very persuasive. And if it's also in your self-interest to listen to him -- and lobbyists are good at nothing if not making sure it is in a politician's long-term self-interest to listen to them -- then all your incentives are pointing in the same direction. You'll listen.
The expression "legislative subsidy" is excellent because it really shows how successful lobbyists hold legislators' hands and help them do their job. This seems very persuasive to me, because living in France, where campaign finance is highly regulated (since a string of corruption scandals in the 1980s), lobbying works very well, as I discovered (full disclosure) when I was in law school and active in politics and briefly worked as a lobbyist. Lobbyists equip politicians with tools to do their job, e.g. position papers, studies on complex topics, introductions to industry and sometimes even media folk, and often even drafted bills. Hence the legislative subsidy.