The Missouri Supreme Court ruled against the state attorney general’s position on an abortion ballot initiative, a decision that allows an effort to restore abortion rights there to move forward.

The court ruled that the attorney general, Andrew Bailey, who opposes abortion, had improperly held up his approval of a ballot initiative that would ask voters whether they want to change the constitution to include a right to abortion. Mr. Bailey, a Republican, disputed the assessment of the state auditor, also a Republican, regarding how much such a change would cost.

The delay prevented proponents of the initiative from beginning to collect signatures to try to place the question on the ballot for next year’s election.

“While today is a tremendous victory for Missourians and the right to direct democracy, it is clear that some who hold office will not hesitate to trample the constitution if it advances their personal interests and political beliefs,” said Luz María Henríquez, the executive director of the American Civil Liberties Union of Missouri.

A spokeswoman for Mr. Bailey said the attorney general’s office disagreed “with the court’s decision, as we believe Missourians deserve to know how much this amendment would cost the state, but we will respect the court’s order.”

Missouri has one of the strictest abortion bans in the country. The procedure is outlawed in nearly all cases beyond medical emergencies. There are no exceptions for rape or incest.

The proposed ballot initiative aims to enshrine the right to make decisions about abortion, birth control, childbirth and other issues related to pregnancy in the state’s Constitution.

Even in conservative states, voters have tended to support abortion rights when asked directly in ballot measures.

As part of the procedure for qualifying for the ballot, the proposal was sent to the state’s auditor, Scott Fitzpatrick, who provided a cost estimate for the proposed initiative.

Mr. Fitzpatrick, who opposes abortion, said the proposal would have an estimated cost of at least $51,000 annually in reduced local tax revenues.

Mr. Bailey rejected that analysis, instead saying in a letter that the bill would most likely cost taxpayers “upward of $12 billion” because of fewer births and the loss of Medicaid funding.

Missouri Supreme Court judges on Thursday unanimously affirmed a lower court’s ruling, writing in their decision that there was no justification for Mr. Bailey’s “refusal to perform the plain, unequivocal and ministerial duty of approving those summaries.”

The attorney general has the authority, the ruling said, only to review the “legal content and form” of the auditor’s reports, “not their substance.”

The attorney general must now approve the auditor’s fiscal assessment within 24 hours, or by 1 p.m. on July 21, and the amendment will then be able to move forward. The proposal will next go to the office of the Missouri secretary of state, who is tasked with certifying the fiscal assessment and a summary of the proposal that would appear on the ballot.

Lawsuits have already been filed by the A.C.L.U. of Missouri against the secretary of state’s proposed wording for the ballot, which the A.C.L.U. says is misleading.

More than 100,000 signatures from voters are needed for the measure to appear on the Missouri ballot next fall, and they must be submitted by early May.