Today, they're paying a call on Pennsylvania Democratic Congressman Chaka Fattah.
"I try to make sure that my door is open to everyone, and that we get the right information so we can make the best decisions," Fattah said.
There are two components to successful lobbying: Access, and money - donations from lobbyists and their clients to Members of Congress. While it's illegal for Members to cast a vote in exchange for campaign contributions (that would be bribery), there's plenty of walking right up to the line.
"I think that when people give campaign contributions, they are not there simply to improve the workings of democracy. They're there to buy access," Thurber said.
"Everybody in Washington who's a lobbyist gives campaign contributions," said Davis. "Almost everybody that I know as a lobbyist. Money itself is not bad. The question [is], is somebody honest or going to be influenced by the money?"
But Professor Thurber says the relationship between lobbyist and Congress is sealed by one guiding principal: "The iron law of reciprocity - meaning, 'I'll help you if you'll help me' - is ingrained in politics. It's ingrained on the Hill. . . . That's perfectly legal. And it's non-transparent, frequently."
The campaign donations and fundraisers often take place out of public view. Earlier this year, we took hidden cameras to a swank, Key Largo resort to observe as lobbyists and other big donors paid thousands of dollars to spend the weekend with Republican Congressmen, mostly freshmen . . . on the golf course . . . charter fishing on a boat named "Good Life" . . . over drinks at the resort bar.
"These freshmen, about half of them said they were aligned with the Tea Party movement, had high ideals about changing Washington, the debt and the deficit, and tax reform and everything, but also about campaign money," said Thurber. "Well, they realized that campaigns are very expensive. And if you want to win, you've got to bring in a lot of money."
Efforts to regulate lobbying date back to 1876, when the House first required lobbyists to register with the Clerk.
In the 21st century, the lobbying industry has become a revolving door for lawmakers. They retire from Congress . . . and make more money returning as lobbyists, often getting special access with their former colleagues on Capitol Hill.
A study by the watchdog group Public Citizen found that 43 percent of Members of Congress who left office between 1998 and mid-2005 went on to register as lobbyists.