Disclosure Data Catalog, Federal Election Commission

"Each of the files listed here can be downloaded in either csv or xml formats. Each also has a metadata page that describes the information included and the structure of the file itself. There is a pdf version of each file if you need to print the information. You can also subscribe to RSS feeds for each of the files so you're notified whenever new data is available or a change is made."

Also see the Commission's Disclosure Data Blog where the FEC will post information about the files and its future plans. And: they say that "you can get help with any questions about the data we're providing here."