Paul Streckfus (Editor, Exempt Organization Tax Review) has published Is the IRS Letting the Heritage Foundation Off the Hook?, 110 Tax Notes 653 (Feb. 6, 2006), also available on the Tax Analysts web site as Doc 2006-1533, 2006 TNT 25-74.  Here is the opening:

The IRS has recently come out with an interesting private letter ruling, LTR 200602042. It probably was sent to the Heritage Foundation, as was an earlier technical advice memo, TAM 200044038, which was reported by the Bloomberg news service in 2000 to involve Heritage. The TAM concluded that a letter sent by Heritage and signed by Bob Dole, who was running against Bill Clinton in 1996, was prohibited political campaign intervention by Heritage on Dole’s behalf, based on its content and timing. In addition, Dole got to use the names and addresses of respondents to the Heritage mailing in his campaign mailings.

LTR 200602042, on the other hand, did not find political intervention when presumably Heritage sent out fundraising letters signed by a U.S. senator or by a representative. The difference in result appears due to timing, out-of-state or out-of-district solicitation (the fundraising letters for Heritage will not be mailed "into the jurisdiction where the signer is or may become a candidate for office"), and no specific mention of an opponent. Also, "nothing in the fund-raising letters suggest or encourages the reader to make a financial contribution to the [senator or representative] for his campaign for office." Nevertheless, the content of the letters in both the TAM and the LTR appears to be clearly political, and one would expect benefit to the signatories, if not locally, then nationally. Obviously, the more national recognition a signatory had, the more benefit. One would expect senators or representatives selected by Heritage or similar organizations to be well-known outside their states or districts.