PHOENIX (AP) — A group opposed to an Arizona law that marked the nation's most ambitious expansion of a private school voucher program said Monday it has collected enough signatures to stop the measure from taking effect this week, allowing voters decide its fate in the 2018 election if the signatures are validated.

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 Education Secretary Betsy DeVos. Opponents view it as an assault on public education, and they began gathering signatures in hopes of repealing the law at the ballot box.

Save Our Schools Arizona has collected more than 100,000 voter signatures and plans to turn them in to the secretary of state's office on Tuesday, spokeswoman Dawn Penich-Thacker said. That's about 25 percent more than required and should be enough to prevent the law from taking effect on Wednesday.

If enough signatures are validated, the law will be on hold until the November 2018 election under Arizona referendum law allowing citizens to challenge legislative action. Voucher backers are likely to challenge the validity of the signatures in hopes of stopping the repeal effort.

The fight could have big consequences for the 2018 election in Arizona. If the group succeeds, they will be seek to drive education-minded voters to the polls in a crucial midterm election. If they fail, Ducey will avoid having to run for re-election on the same ballot where his signature education effort is being challenged.

Lawmakers also could move to sidetrack the election by repealing the law, a tactic they used the last time voters succeeded in blocking a law enacted by the Legislature. That involved an election law passed in 2013 and repealed the next year.

The expansion law will be immediately blocked if the group turns in more than the 75,321 required signatures under Arizona's voter referendum law, said Matt Roberts, spokesman for Secretary of State Michele Reagan. Tuesday's expected filing will kick off a process lasting more than a month where the secretary of state and county recorders verify the signatures.

Penich-Thacker said the group anticipates legal challenges from voucher backers, something that has become common from opponents of voter initiatives in recent years.

Kim Martinez, Arizona spokeswoman for the school choice group American Federation for Children, said she doubts Save Our Schools has enough valid signatures.

"It appears to us that they fell short," Martinez said. "From the beginning they themselves in their own words predicted they would need about 120,000 signatures."

Martinez declined to specifically say if her group would go to court to challenge signatures. But she did say they plan to scrutinize "every single signature on every petition."

"Of course we're going to make sure all the signatures are valid, and we are going to continue to fight for a program that empowers students and parent who need it," Martinez said.

Since Arizona first passed a voucher program for disabled students in 2011, it has been repeatedly expanded and now covers about 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.

Voucher opponents say that's because low-income parents can't afford to send their children to private schools despite the state cash, prefer neighborhood public schools or can't afford to drive their children to private schools.

Technically called Empowerment Scholarship Accounts, the Arizona program allows parents to take between 90 percent and 100 percent of the state money a local public school would receive to pay for private or religious education. The average student who isn't disabled currently receives about $6,000 a year to pay for tuition or other costs, while disabled students get about $20,000.

The new expansion would go into effect for the fall semester if opponents fall short during the signature validation process. It expands eligibility to all students by 2022 but caps enrollment at about 30,000.

Supporters say vouchers give parents more choice. Opponents argue they siphon money from public schools.