Proposition 38 would give parents $4,000 per pupil to send their children to private schools, including religious ones. Parents would get the vouchers regardless of family income.
The issue has stirred controversy, with critics saying public school education would be destroyed and the move would utterly violate the constitutional separation of church and state.
Congress has taken up the issue within the past year, and presidential candidates have been asked for their stances.
Supporters say children's futures are at stake.
"There are millions of kids trapped in really bad schools with teachers who tell them they can't be anything," California ballot sponsor Tim Draper said in his campaign bus outside a suburban Sacramento Christian school. Draper, a venture capitalist and former member of the state Board of Education, has put his four children in private schools.
The prospect of a mass exodus from California's public schools comes as opponents cite improvement efforts by Gov. Gray Davis since his 1998 election. Achievement test scores released this month showed gains in nearly all grades and subjects, though students remain below the national average in reading.
"The programs are beginning to work," Davis said Tuesday. "My message is to stay the course. The voucher is a 180-degree mistake."
Voters have rejected similar initiatives in Colorado and Washington, as well as California, said Martha McCarthy, an education professor at Indiana University.
California's initiative is one of two before voters this November, along with one in Michigan.
"A solid win, particularly in a state like California, I don't think would do anything but provide an impetus" for other states, McCarthy said.
California voters overwhelmingly rejected a 1993 measure to give parents a $2,600 voucher. But voucher supporter Alan Bonsteel of California Parents for Educational Choice said things have "gone downhill dramatically" since then.
Nearly 12,000 children in Milwaukee, Cleveland and Florida attended private schools last year using vouchers. All three programs limit the vouchers - Milwaukee's and Cleveland's to low-income children and Florida's to students in schools with two failing state ratings within four years. More than 70 percent of children in the Milwaukee and Cleveland programs are black or Hispanic.
Proposition 38 opponents say it would prohibit voucher schools from discriminating on the basis of race or ethnicity, but let them exclude students based on their language, religion, ability to pay or academic or physical abilities.
"Proposition 38 is a cruel hoax, especially for ethnic minority children," said Leon Beauchman, a member of the Santa Clara County Board o Education and a director of the California School Boards Association.
School groups opposing the measure began airing television ads Tuesday, one in English featuring Davis and one in Spanish with Lt. Gov. Cruz Bustamante. Both say the initiative would give money to private schools not subject to state standards.
Draper has pledged to spend $20 million of his own money to match the $20 million he expects to be spent on ads by opponents, who include virtually everyone in the state's education establishment.
He has been unable to attract support, however, from private schools, which might be expected to back the ballot measure, and is opposed by a conservative taxpayer group that backed the 1993 plan.
Proposition 38 could cost taxpayers if few public school students transfer to private schools, and all the students now in private schools collect vouchers, said Jon Coupal, president of the Howard Jarvis Taxpayers Association.
Draper is focusing much of his campaign on minority parents, particularly those whose children attend decaying and low-performing urban schools.
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