Two conservative taxpayer advocacy groups filed suit Wednesday against Denver over campaign finance disclosure rules that they say will violate the privacy rights of their donors when the groups get involved in city elections.

The lawsuit, filed by the Phoenix-based Goldwater Institute on behalf of the two groups, says changes approved by the City Council in September violate the free speech provision of the First Amendment. The city ordinance requires clubs, associations, corporations and groups that advocate for or against local ballot measures to meet the disclosure requirements of issue committees once they raise and spend at least $500.

Once it passes that threshold, an issue committee must identify by name and address each donor who gave $50 or more within that calendar year.

“We have donors who like to remain anonymous, and we’d like to honor their requests,” said Marty Neilson of the Colorado Union of Taxpayers. “We think this is an unconstitutional ordinance.”

The lawsuit was filed in Denver District Court. Also joining as a plaintiff is the TABOR Committee, which advocates for protecting the state’s voter-passed Taxpayer’s Bill of Rights.

A broader definition for issue committees, along with a reporting schedule, were part of a package of campaign finance changes that are set to take effect Jan. 1. When the council considered the proposed changes, more attention was focused on the addition of spending and donor reporting requirements for independent expenditures and electioneering communications that favor or oppose candidates and ballot issues. But those latter provisions aren’t targeted by the suit.

Most of the changes were aimed at increasing transparency on money in local elections, Clerk and Recorder Debra Johnson said at the time.

The city attorney’s office deferred comment to the Denver Elections Division, which declined through a spokesman to comment on pending litigation.

The legal challenge, in targeting disclosure requirements’ impact on nonprofit groups, raises an issue that goes back more than a decade in Colorado, pitting donor privacy against the interest of campaign transparency.

In 2008, the Colorado Court of Appeals upheld a similar state law in a case brought by the libertarian-leaning Independence Institute. The court found that the law, which required disclosure of donors who gave $20 or more once a nonprofit spent $200 on election advocacy, was appropriate.

The Goldwater Institute, which has challenged similar laws in other states, argues that two more recent rulings by the 10th U.S. Circuit Court of Appeals in Denver have changed the legal landscape.

In the more recent of the two, the court ruled in early 2016 that the burdens imposed by the law on the nonprofit Coalition for Secular Government, which expected to spend $3,500 challenging a proposed “personhood” amendment, were greater than the “government’s modest informational interest” in requiring donor disclosure.

The state legislature reacted to that ruling by exempting “small-scale” issue committees that raise or spend up to $5,000 from disclosing their donors. Until they reach that threshold, groups must only register as a committee.

Matt Miller, an attorney for the Goldwater Institute, argues that a requirement for donor transparency makes more sense when a group is advocating for or against a candidate, given concerns about corruption. But that concern is less common when the passage of a ballot measure is at stake, he said.

“The 10th Circuit has made it very clear that under the First Amendment, the government’s interest in this information is very minimal,” Miller said. “So we now believe that (the 2008 state ruling) was wrongly decided.”