Can a private non profit organization require its members (a senior center) be American citizens?
An organization may legally discriminate for some reasons but may not legally discriminate for other reasons. There are federal laws that apply to certain businesses (whether for-profit or nonprofit). Generally a senior citizen center is subject to the public accommodations laws:
(page 3 at (K))
Section 201 of the Civil Rights Act of 1964 provides, "All persons shall be entitled to the full and equal enjoyment of the goods, services, facilities, and privileges, advantages, and accommodations of any place of public accommodation, as defined in this section, without discrimination or segregation on the ground of race, color, religion, or national origin." http://goo.gl/PAsL5C
A citizenship requirement is not the same as a national origin requirement. The Idaho Commission on Human Rights explains:
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"National origin" and "citizenship" are different legal concepts and are treated differently under the law
"National origin" refers to the country from which an individual or her/his forbears came; it does not refer to whether the person is a U.S. citizen. Anti-discrimination laws protect citizens and non-citizens alike from discrimination based on race, national origin, or color. However, the laws enforced by the IHRC do not protect an individual who believes that he/she has been discriminated against because of the particular citizenship held: U.S. citizen or citizen of another country.
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(2nd paragraph from the bottom).
I have in my profile that this free forum is only for general questions about IRS federal exemption issues of 501(c)(3) organizations. Your matter also is controlled by state law and states sometimes have more expansive civil rights laws. Even though this forum is not for such state law issues, it appears that Alaska does not have a law that prohibits discrimination on the basis of citizenship.
Section 18.80.230 of the Alaska Code provides:
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It is unlawful for the owner, lessee, manager, agent, or employee of a public accommodation
(1) to refuse, withhold from, or deny to a person any of its services, goods, facilities, advantages, or privileges because of sex, physical or mental disability, marital status, changes in marital status, pregnancy, parenthood, race, religion, color, or national origin;
(2) to publish, circulate, issue, display, post, or mail a written or printed communication, notice, or advertisement that states or implies
(A) that any of the services, goods, facilities, advantages, or privileges of the public accommodation will be refused, withheld from, or denied to a person of a certain race, religion, sex, physical or mental disability, marital status, color, or national origin or because of pregnancy, parenthood, or a change in marital status, or
(B) that the patronage of a person belonging to a particular race, creed, sex, marital status, color, or national origin or who, because of pregnancy, parenthood, physical or mental disability, or a change in marital status, is unwelcome, not desired, or solicited.
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Therefore, unless your city prohibits discrimination on the basis of citizenship, a private non profit organization that runs a senior center may require its members to be U.S. citizens.
Attorney at Law
P.S. This response is intended to be a general statement of law, should not be relied upon as legal advice and does not create an attorney/client relationship.