Following up on this post, my colleague Ellen Aprill offers these thoughts:

The basic rule is that lobbying and political expenditures must come out of after-tax money (not pre-tax, that is, not deductible, money). There is a narrow exception to this basic rule in that veterans’ organizations are permitted to make lobbying and political expenditures and contribution to such organization are tax-deductible. There is a somewhat broader exception for lobbying in that s 501(c)(3) organizations, popularly known as charities, can engage in a limited amounts of lobbying and contributions to them are tax-deductible.
The rule of using only pre-tax money for lobbying and political expenditures is achieved for businesses, whether or not incorporated, because the general provision that allows deductions for trade and business expenses, s 162 has a special rule in subsection (e) disallowing a deduction for lobbying and political expenditures (with another special rule allowing the deduction for certain cases of local legislation). The relevant provision, s 162(e), is quoted below.
Nowhere in Citizens United does the Court question the denial of the deduction for corporate lobbying expenditures or suggest that it is an unconstitutional burden. (Admittedly, neither does the Court acknowledge the denial of the deduction for these expenditures.) Prior to enactment of s162(e), the Supreme Court in Cammarano v. U.S., 358 U.S. 498 (1959), rejected a taxpayer’s First Amendment challenge to IRS regulations denying deductions for business lobbying expenditures.
These s162(e) rules have a particularly convoluted impact on trade associations, which are exempt under s501(c)(6). Trade associations must calculate and report on their annual Form 990 Information Return (which is publicly available) their total s162(e) expenses and the portion of their membership dues allocable to those expenses. They must notify dues paying members at least annually of the amount that is not deductible. Instead of making this notification, the organization has the option of itself paying a “proxy” tax at the highest corporate rate on the total amount of its s162(e) expenditures. On its 2008 Form 990, for example, the U.S. Chamber of Commerce reported that it notified its members that $64,953.320 of $118,645,920 in member dues and assessments were nondeductible under s162(e). The D.C. Circuit upheld this set of trade association rules against First Amendment challenge in American Society of Association Executives v. United States, 195 F.3d 47 (D.C.Cir. 1999).
Section 162(e)’s text is below the fold.

(e) Denial of deduction for certain lobbying and political expenditures
(1) In general
No deduction shall be allowed under subsection (a) for any amount paid or incurred in connection with—
(A) influencing legislation,
(B) participation in, or intervention in, any political campaign on behalf of (or in opposition to) any candidate for public office,
(C) any attempt to influence the general public, or segments thereof, with respect to elections, legislative matters, or referendums, or
(D) any direct communication with a covered executive branch official in an attempt to influence the official actions or positions of such official.
(2) Exception for local legislation
In the case of any legislation of any local council or similar governing body—
(A) paragraph (1)(A) shall not apply, and
(B) the deduction allowed by subsection (a) shall include all ordinary and necessary expenses (including, but not limited to, traveling expenses described in subsection (a)(2) and the cost of preparing testimony) paid or incurred during the taxable year in carrying on any trade or business—
(i) in direct connection with appearances before, submission of statements to, or sending communications to the committees, or individual members, of such council or body with respect to legislation or proposed legislation of direct interest to the taxpayer, or
(ii) in direct connection with communication of information between the taxpayer and an organization of which the taxpayer is a member with respect to any such legislation or proposed legislation which is of direct interest to the taxpayer and to such organization,
and that portion of the dues so paid or incurred with respect to any organization of which the taxpayer is a member which is attributable to the expenses of the activities described in clauses (i) and (ii) carried on by such organization.
(3) Application to dues of tax-exempt organizations
No deduction shall be allowed under subsection (a) for the portion of dues or other similar amounts paid by the taxpayer to an organization which is exempt from tax under this subtitle which the organization notifies the taxpayer under section 6033 (e)(1)(A)(ii) is allocable to expenditures to which paragraph (1) applies.
(4) Influencing legislation
For purposes of this subsection—
(A) In general
The term “influencing legislation” means any attempt to influence any legislation through communication with any member or employee of a legislative body, or with any government official or employee who may participate in the formulation of legislation.
(B) Legislation
The term “legislation” has the meaning given such term by section 4911 (e)(2).
(5) Other special rules
(A) Exception for certain taxpayers
In the case of any taxpayer engaged in the trade or business of conducting activities described in paragraph (1), paragraph (1) shall not apply to expenditures of the taxpayer in conducting such activities directly on behalf of another person (but shall apply to payments by such other person to the taxpayer for conducting such activities).
(B) De minimis exception
(i) In general Paragraph (1) shall not apply to any in-house expenditures for any taxable year if such expenditures do not exceed $2,000. In determining whether a taxpayer exceeds the $2,000 limit under this clause, there shall not be taken into account overhead costs otherwise allocable to activities described in paragraphs (1)(A) and (D).
(ii) In-house expenditures For purposes of clause (i), the term “in-house expenditures” means expenditures described in paragraphs (1)(A) and (D) other than—
(I) payments by the taxpayer to a person engaged in the trade or business of conducting activities described in paragraph (1) for the conduct of such activities on behalf of the taxpayer, or
(II) dues or other similar amounts paid or incurred by the taxpayer which are allocable to activities described in paragraph (1).
(C) Expenses incurred in connection with lobbying and political activities
Any amount paid or incurred for research for, or preparation, planning, or coordination of, any activity described in paragraph (1) shall be treated as paid or incurred in connection with such activity.
(6) Covered executive branch official
For purposes of this subsection, the term “covered executive branch official” means—
(A) the President,
(B) the Vice President,
(C) any officer or employee of the White House Office of the Executive Office of the President, and the 2 most senior level officers of each of the other agencies in such Executive Office, and
(D)
(i) any individual serving in a position in level I of the Executive Schedule under section 5312 of title 5, United States Code,
(ii) any other individual designated by the President as having Cabinet level status, and
(iii) any immediate deputy of an individual described in clause (i) or (ii).
(7) Special rule for Indian tribal governments
For purposes of this subsection, an Indian tribal government shall be treated in the same manner as a local council or similar governing body.
(8) Cross reference
For reporting requirements and alternative taxes related to this subsection, see section 6033 (e).