What was the reason for the drastic increase in congressional staff during the 20th century?

Congressional staff increased greatly in the 20th century due to the shift in politics from grassroots to higher levels of lobbying, groups and special interests. The congress person was not only invested in their district but in those special interest groups requesting meetings and events with the congressperson. By increasing staff, they would be able to learn more about these groups and lobbyists so they could make a more informed decision. It also allowed the staff to deal directly with the public. A congressman in the early 20th century would only have in person visits and the occasional phone call, but by the end of the 20th century, the congressperson would have contact via appointment, call, letter, email and even rudimentary instant contact via Instant Messenger and sites like Myspace.