This Seattle Times investigative reporting series used public records to show a link in about half the cases between special pork-barrel federal budget expenditures (called "earmarks") and campaign contributions. (I thought the "new" Congress elected in 2006 was going to ban earmarks in the first 100 hours. Gee, what happened?)
Here is the Times story behind the story explaining how the investigative reporters used public records to obtain this information and then make the linkage. Excellent work.