There's a good argument to be made about the 1970 Legislative Reorganization Act as being one of the first places to look. Up until then, Congress voted on a secret ballot, and the congressional votes were not made public - so lobbying had a much higher hurdle to clear (since the lobbyists could not guarantee a return on their investment by verifying that the Congressperson kept their end of the bargain).

https://www.youtube.com/watch?v=c4kvUxQIJlA