"Follow the money." That was the advice given to journalists Woodward and Bernstein in the 1976 film All the President's Men, as they unraveled the machinations behind Watergate. While an ordinary citizen's motives may be less sensational, we all have the right to know where our government officials are getting their financial support. The New York Times is helping enable that research with their Campaign Finance API.
Originally launched in October of 2008 with data for that year's Presidential candidates, the new version 3--released just in time for the 2010 midterm elections--now include House and Senate election candidates. Data comes straight from Federal Election Commission (FEC) filings, which are searchable online but with an interface not especially conducive to automation. The Times' Campaign Finance API collects all that public information and lets registered users query by year, candidate, donor, location, and other parameters. Results are returned in either JSON or XML format.
Of course, the FEC being a government operation, there are still a few speed bumps. Daily electronic filings of Congressional candidates' and political action committees' (PAC) fundraising information became available starting in 2009, and the API refreshes those data every fifteen minutes--except for the Senate, because that august body still files everything on paper. (Think this should change? Contact your state's Senators.)
In general, Congressional data back to the year 2000 is available, and Times developers are working on adding earlier data. When combined with the Times' other data resources, like the Congress API and the Districts API, interested citizens have a myriad of opportunities for political research and investigation.
There's a lot of data available here--"thousands of candidates and committees in each [election] cycle," according to the Times developer blog--so the Campaign Finance API offers search methods for both. API clients are limited to 5,000 requests per day, and use of the data is restricted by the FEC's sale and use regulations (PDF download link). The API documentation includes plenty of examples to help you get started, and the Times welcomes your questions and suggestions for future improvements in its Developer Forums.