Via the Washington Post, an example of some of the pressure Georgia Senators Chambliss and Isakson were under:

Isakson's office received more than 21,000 calls from opponents of the bill, compared with 6,000 from supporters.

Also, a hint of some of the back-room pressure from the White House (via The Hill):

The vote was a major setback to the White House, which dispatched its top two emissaries — Commerce Secretary Carlos Gutierrez and Homeland Security Secretary Michael Chertoff — to twist the arms of senators entering the chamber just prior to the vote.