PHOENIX (AP) - A group opposed to a major private school voucher expansion bill signed by Arizona Gov. Doug Ducey says it is on track to collect enough signatures to block the law.

Save Our Schools spokeswoman Dawn Penich-Thacker says the group isn't releasing the number of signatures it has collected so far. But she said the all-volunteer effort should collect well above the minimum of 75,000 signatures by the Aug. 8 deadline.

RELATED: Where will Arizona's education dollars be spent under school voucher bill?

If the effort succeeds, the voucher expansion will be put on hold until the November 2018 election.

Arizona started a voucher program in 2011 and it now covers about a third of all 1.2 million state students. Still, only about 3,500 now get vouchers.

The new law expands eligibility to all students by 2022 but caps enrollment at about 30,000.