The voter who filed the case alleges that 69 people who collected signatures for the proposed graduated income tax measure were not properly trained or registered. If all signatures collected by those circulators were thrown out, the lawsuit argues, the measure would have 121,089 signatures — too few to qualify for the ballot.

Colorado requires paid petition circulators to complete an online training course, and the organization sponsoring a petition must register its circulators with the state. In its response to the lawsuit, the Secretary of State’s office says registration failures are not grounds for invalidating signatures; the petition organization could instead face a fine or lose its license. The office also says 33 of the circulators identified by plaintiffs were registered, and that counting their signatures would leave Amendment 87 eligible for the ballot.

J.J. Ament, president and CEO of the Denver Metro Chamber of Commerce, which opposes the amendment, said the signature-gathering process should protect its integrity and limit opportunities for fraud. He is not a plaintiff. The measure’s campaign disputed the challenge’s motives: “It’s no surprise that wealthy interests would rather fight Amendment 87 in court than in front of voters,” said Katie Farnan, the campaign’s communications director.

Amendment 87 proposes a graduated income tax, with the campaign describing it as a tax increase for people earning more than $500,000. With ballots already printed and mailed, the court dispute could determine whether voters’ choices on the measure are counted, even as the state maintains that the signature total meets the requirements.