The Congressional Research Service has released Political Organizations Under Section 527 of the Internal Revenue Code (RS21716), also available on the Tax Analysts web site as Doc 2005-3455, 2005 TNT 34-44. Here is the Summary:

Political organizations have the primary purpose of influencing federal, state, or local elections and conducting similar activities. Those that qualify under section 527 of the Internal Revenue Code are taxed only on certain income. Under the Code, 527 organizations are subject to reporting requirements that involve registration, the periodic disclosure of contributions and expenditures, and the annual filing of tax returns. Section 527 organizations must also comply with applicable campaign finance laws. This report will briefly describe these organizations and the reporting requirements they face under the Code. The report will be updated as events warrant.