A brief yet interesting (in a dull way) definition and history of 527 organizations can be found here:
http://www.brookings.org/gs/cf/headlines/527_intro.htmI. Introduction
A political organization under the Internal Revenue Code (26 U.S.C. § 527) is defined as a party, committee, association, fund, or other organization (whether or not incorporated) organized and operated primarily for the purpose of directly or indirectly accepting contributions or making expenditures, or both, for an exempt function. Id. The "exempt function" of a "political organization" is defined as "influencing or attempting to influence the selection, nomination, election or appointment of an individual to a federal, state, or local public office or office in a political organization. . . ." § 527(e)(2).
II. Brief History of 527 Political Organizations
Section 527 was added to the Internal Revenue Code ("Code") in 1974 to provide an exemption from federal income tax and gift tax to "political organizations." In creating Section 527, Congress reasoned that campaigns, party committees, and PACs should not pay taxes on funds contributed to such political entities and used for political purposes. Presumably because these political organizations already registered and reported their contributions and expenditures to the Federal Election Commission ("FEC") or state agencies, Congress created no special registration and disclosure provisions for Section 527 political organizations at the Internal Revenue Service ("IRS"). In fact, the only report they were required to file with the IRS was a (confidential) tax return.
In 1996, the IRS was asked whether an "issue advocacy" organization that was not registered as a candidate committee, party committee, or PAC could qualify for Section 527 "political organization" tax exempt status if its principle purpose was to influence elections. In several private letter rulings, the IRS said that groups seeking to influence elections through candidate-specific issue advertising would qualify as political organizations, regardless of whether they were registered with the FEC or state election agencies. The IRS reasoned that Congress intended Section 527 of the tax code to cover all political entities, whether or not their activities also fell within federal or state election law. The IRS also apparently was motivated by a desire to have election-influencing activities done by Section 527 groups, rather than by Section 501(c)(4) organizations.
The problem with this approach became all too apparent in the 1998 election cycle and the 2000 presidential primaries: advertising that attacked specific candidates (but avoided using "magic" words such as "vote for" or "oppose") from new Section 527 political organizations with generic names like "Americans for Better Government" flooded elections. These groups (frequently called "Stealth PACs") often had no apparent existence beyond a registered agent and a post office box and filed no registration statements. Accordingly, their officers and contributors (and often even motives) were a mystery.
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For a look at 527 organizations from a tax standpoint and presented in a non-technical way can be found here:
http://www.irs.gov/pub/irs-drop/rr-04-6.pdf26 CFR 1.527-2: Definitions.
(Also § 501.)
Rev. Rul. 2004-6
Organizations that are exempt from federal income tax under § 501(a) as organizations described in § 501(c)(4), § 501(c)(5), or § 501(c)(6) may, consistent with their exempt purpose, publicly advocate positions on public policy issues. This advocacy may include lobbying for legislation consistent with these positions. Because public policy advocacy may involve discussion of the positions of public officials who are also candidates for public office, a public
policy advocacy communication may constitute an exempt function within the meaning of § 527(e)(2). If so, the organization would be subject to tax under § 527(f).
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Doing a search using the following will provide lots more info.
26 C.F.R. §§ 1.527-2(a)(2)-(3)