PHOENIX — A conservative advocacy group founded by the Koch brothers is asking a federal judge to quash a new voter-approved campaign finance law aimed at exposing “dark money’’ contributions for political purposes.

Americans for Prosperity contends the First Amendment protects the right of individuals to donate to advocacy organizations without fear their identities will be disclosed.

In a new lawsuit, they say Proposition 211 “trammels that right by subjecting countless Americans nationwide to governmental doxxing for doing nothing more than supporting their chosen non-profit organizations and charities.’’ Doxxing means to publicly identify or publish private information about someone.

The group, founded by billionaire conservative activists Charles and David Koch, wants U.S. District Court Judge Roslyn Silver to block the Citizens Clean Elections Commission from enforcing the law, which was approved by Arizona voters by a nearly 3-1 margin in November.

The lawsuit is the second effort by special interest groups to kill the law.

A separate lawsuit filed by the Arizona Free Enterprise Club and the Center for Arizona Policy is pending in Maricopa County Superior Court. It contends the initiative runs afoul of a state constitutional right of privacy. The judge in that case is set to hear arguments in May.

Donors may fear reprisals

Voters in November closed what some consider to be a loophole in state campaign finance laws.

Arizona has for years required that the names be made public of those who give at least $50 to political campaigns, or to support or oppose ballot measures. That includes “in-kind’’ contributions, where organizations, rather than giving money to a cause, run their own commercials.

But that law said only the name of the organization need be made public, not the individuals or corporations who donated to the group. Many commercials identify the sponsors by only a bland-sounding organizational name, with viewers given no clue as to who was really financing them.

Proposition 211 requires public disclosure of anyone who has given at least $5,000 to one of these front groups. Potentially more significant, it requires any group making political expenditures to trace the cash back to its original source, no matter how many hands it passed through.

Americans for Prosperity and its affiliated foundation say that is harmful to the organization and its donors, who may fear reprisals.

“Some people publicly associated with the plaintiffs have faced boycotts, character attacks, personal threats, and worse as a result,’’ its attorneys told Silver. “Others simply have no desire for their giving to be made public.’’

Many donors have insisted their identities remain confidential, the lawsuit says.

“This assurance of confidentiality is vitally important and enables plaintiffs’ continued robust participation in the public sphere — whereas Proposition 211’s compelled disclosures vitiate this understanding and threaten to chill continued donations and support for plaintiffs,’’ it says.

The group says there are other problems. The lawsuit says the initiative used the phrase “campaign media spending’’ as the trigger for when disclosure is required. That means not just any public communication that promotes, supports, attacks or opposes a candidate within six months of an election, the suit says, but also any communication that merely “refers to a clearly identified candidate’’ between 90 days before a primary right through the general election.

“The law lacks guardrails to ensure that the speech it regulates has a sufficient nexus to the asserted interests, specific to the electoral context, that Arizona purports to have in the required donor information,’’ the lawsuit states. “The unduly broad reach of Proposition 211 ventures far beyond anything the First Amendment permits.’’

‘Confuses voters’

The group’s lawyers have another legal theory.

They note that Proposition 211 was advertised as providing Arizonans with needed information, even to the point of being dubbed by its sponsors as the “Voters’ Right to Know Act.’’ But the net effect, they argue, would be just the reverse.

“Far from improving voters’ knowledge, Proposition 211 stands affirmatively to mislead interested voters by directly tying donors to candidates and issues those donors may support only glancingly, or not at all,’’ the complaint alleges.

Anyway, it contends, the sheer volume of information about donors that Proposition 211 would produce wouldn’t help “given voters’ likely inability to appreciate any contributors beyond direct donors.’’

The measure ties the names of individuals and corporations who give to a group for what could be “a plethora of reasons’’ to everything that group does in the future, the lawsuit says.

“Proposition 211 chills protected speech and confuses voters, in violation of the First Amendment,’’ the lawsuit states.

There was no immediate response to this suit.

But in legal filings in the previously filed state court case, attorney Chanele Reyes, representing the sponsors of Proposition 211, said there is nothing unconstitutional about ensuring that voters know who is trying to influence elections.

Reyes pointed out the state has had financial disclosure laws for years, all of which have been upheld by multiple courts. All Proposition 211 does, Reyes said, is pull back the curtain to ensure that voters know not just the name of an organization listed as the direct source of money, but the individuals, organizations and corporations who are really providing the financing.

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Howard Fischer is a veteran journalist who has been reporting since 1970 and covering state politics and the Legislature since 1982. Follow him on Twitter at @azcapmedia or email