Just in time for next week's mid-term US elections, just listed here is this API from The Institute on Money in State Politics. And in this case, it's the domain name that's more telling: followthemoney.org. It's quite an impressive store of data on the who, where, when and how much of campaign contributors across the US. From their site:
Money in state politics plays a pivotal role in shaping public policy in individual states and across the nation. The nonpartisan Institute on Money in State Politics tracks contributions in all 50 states and makes this data easily searchable online...The Institute collects campaign-finance data for state-level candidates, party committees and ballot-measure committees in all 50 states. Each two-year election cycle, data-acquisition specialists compile more than 90,000 disclosure reports from more than 16,000 candidates in the states and process more than 3.2 million records of contribution information.
There are about 30 API methods and are grouped by function: states, candidates, party_pacs, ballot_measures, and contributors. For code samples, use their interactive API tester and code generator. They even offer a handy Widget Builder so you can place a widget on your site that displays live data about your (least) favorite candidate or PAC (political action committee). You'll need to sign-up for an API key in order to use this, but it is a fairly painless process. Below is a screenshot of an example widget, in this case the Economic Sector Breakdown for Arnold Schwarzenegger in California for 2003.
For an earlier good mashup on visualizing where political money comes from, see Following Political Dollars.