Nonprofit groups are a force in U.S. politics. They may not fly a campaign's colors or mention a candidate by name, but "soft money" organizations clearly take sides and throw around their weight and money.

The most well known tax-exempt political groups are 527s, named for the section of applicable U.S. tax code. However, other tax-exempt groups also influence elections.

In "The Secret Money Project," NPR and the Center for Investigative Reporting have set out to report on how independent groups influence the 2008 election.

"So far, nobody has produced a roundhouse-punch of an ad like the Swift Boat Veterans ads that floored Democratic Sen. John Kerry's presidential bid four years ago," reports Peter Overby in the project's latest installment, posted July 18. "But over the past week or so, the independent ads have begun getting sharper and more numerous."

A June 25 story shows how Progress Now and a handful of similar state groups leverage public relations and e-mail to push their political agendas.

How do you learn more about these groups, who they are, who gives them money and who gets their money?

• The IRE is regularly updating a database of 527s on file with the Internal Revenue Service, major contributors and major expenses. The IRE charges a fee to access this data, which require some CAR skills to parse. Also, IRE's Campaign Finance Information Center compiles resources for journalists covering money in politics, as well as recent news stories about political finance.
• The Center for Responsive Politics follows 527s and their activities. The center offers a free, easy-to-use tool to look up groups and contributors. Check out its map to find 527s registered in your state. (See my earlier blog post about the wonders of the center's Web site.)
• GuideStar collects tax returns for many 501(c) nonprofits. Anyone who signs onto the site can search and access three years of an organization's filings. A basic GuideStar Premium subscription - $40 for five days, $100 per month and $1,000 per year - gives you access to many more records.