Utahns for Public Schools spokeswoman Lindsay Zizumbo is tight-lipped about the progress of the petition drive.

"We feel really uncomfortable right now giving out estimates," she said Friday. "We will be collecting and counting signatures until the time of the announcement Monday morning."

The group has scheduled a Capitol news conference for Monday at 11:30 a.m.

Early estimates suggested about 20 percent of signatures could be disqualified because they don't belong to registered voters. Zizumbo said signature estimates were changing so quickly Friday she could not speculate on the number collected so far.

Utahns for Public Schools is a political issue committee organized after the 2007 legislative session to oppose the state's new school voucher law. The Parent Choice in Education Act allows parents to apply for private-school tuition assistance from the state. Tuition vouchers range from $3,000 for the poorest families to $500 for the richest.

Voucher supporters have worked to pass such legislation since at least 2000. This year, they mustered enough support to clear the House by a single vote. That's when opponents began exploring legal options to block the measure.

They filed a referendum petition application to try to collect enough signatures to put the law before voters. The group has through Monday to collect 92,000 signatures - representing 10 percent of voters from the last gubernatorial election in at least 15 counties.

If the drive succeeds, the Parent Choice in Education Act will be shelved until the public votes whether to repeal it. The election date will be decided by Gov. Jon Huntsman Jr. and an election could take place as soon as June.

If the petition drive fails, the law could take effect this fall.

But potential legal challenges lurk. If the referendum effort fails, voucher opponents likely will mount a constitutional challenge. If it passes, voucher supporters are expected to ask courts to rule that a second voucher bill supersedes the shelved one. Attorney General Mark ShurtÂleff's office evaluated both bills and concluded that a second bill amending the original act could likely stand on its own and allow Utah to start a voucher program regardless of the referendum outcome.

Either side could challenge the petition drive's results.

"I fully expect a court challenge no matter how this turns out," Shurtleff said last month. "There probably will be a fight over even how many signatures are gathered."

For now, all eyes are on the petitions. The organizers planned to collect signatures all weekend and work to retrieve petition booklets handed out last month.

"We're doing a mass push this weekend," Zizumbo said. "We're on the phones, out in the community and working . . . to get those packets back."

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* NICOLE STRICKER can be reached at nstricker@sltrib.com or 801-257-8999.

Referendum timeline

Referendum organizers for both the school voucher and RSL petitions are running out of time to gather valid signatures.

* MONDAY: All petitions due to county clerk offices by 5 p.m.

* APRIL 24: County clerks must certify registered voters' signatures and deliver petitions to the Lieutenant Governor's Office by 5 p.m.

* APRIL 30: Lieutenant Governor's Office has until 5 p.m. to count the valid signatures and deem the petition drive "sufficient" or "insufficient."

Source: Lieutenant Governor's Office staff