PHOENIX — Backers of Arizona’s ambitious school voucher expansion on Friday launched their first legal challenges to a grassroots signature-collection effort that succeeded in at least temporarily blocking the law.

Lawyers for Americans for Prosperity sent a letter to state Elections Director Eric Spencer saying that a fatal flaw in the introduction to the petition sheets requires him to disqualify all of the more than 111,000 signatures filed earlier this week.

The group said the introduction incorrectly referred to this year’s legislative session, giving it an improper legal description.

“Because this inaccuracy renders the Referendum Petition not strictly compliant with (state law), it is legally insufficient,” attorney Thomas Basile wrote. The letter asks Spencer to “disqualify every sheet of the Referendum Petition containing the error.”

Spencer said all the petition sheets contained the purported error. He said he was skeptical that it is within the Secretary of State’s authority to address the issue Basile raised.

Separately, a lawsuit seeks to toss out petitions that were allegedly circulated by a handful of people who either didn’t properly register as paid signature gatherers or are convicted felons and legally disqualified. The number of signatures collected by those people is unclear.

A grassroots group called Save Our Schools Arizona that was organized when the voucher law was passed collected the signatures. A spokeswoman for the group didn’t immediately return a call seeking comment Friday night, but a tweet from the group promised a battle.

“Rest assured: We’re not surprised by frivolous complaints from dark money foes,” the tweet said. “Our expert attorneys are fighting to GIVE AZ VOTERS OUR SAY.”

The actions come as the Secretary of State’s office has begun the petition verification process, all the while watched by attorneys and volunteers for the school choice groups that backed the new universal voucher law.

State and county officials will determine if slightly more than 75,000 valid signatures were collected. If they are certified, the law remains blocked until the November 2018 general election under an Arizona law allowing voters to weigh in on laws passed by the Legislature.

Voucher backers say they give parents more choice, while opponents argue they siphon money from cash-starved public schools.

Legal challenges have been expected from the school choice groups that backed the measure. Americans for Prosperity is a conservative group funded by billionaire brothers Charles and David Koch. The school choice group American Federation for Children, formerly led by now-Education Secretary Betsy DeVos, lobbied fiercely for the law in this year’s legislative session.

The law making private school vouchers available to every student in the state was a top legislative priority of Republican Gov. Doug Ducey and had the firm backing of the Trump administration and DeVos. The new expansion would go into effect for the fall semester if opponents come up short during the signature validation process.

It expands eligibility to all students by 2022, but it caps enrollment at about 30,000.

Arizona first passed a voucher program, technically called Empowerment Scholarship Accounts, for disabled students in 2011. It now covers a third of all students, including children attending failing schools, those living on Indian reservations, foster children and children of military members. Despite those changes, only about 3,500 students now use it to pay for private school tuition, and more than half are disabled.