San Francisco ? Silicon Valley tycoons, Nobel laureates and Hollywood celebrities are backing a measure on California’s Nov. 2 ballot to devote $3 billion to human embryonic stem cell experiments in what would be the biggest-ever state-supported scientific research program in the country.

The measure — designed to get around the Bush administration’s restrictions on the funding of such research — would put California at the very forefront of the field. It would dwarf all current stem cell projects in the United States, whether privately or publicly financed.

Proposition 71 promises to be one of the most contentious election issues in California, pitting scientists, sympathetic patients who could benefit from stem cells and biotechnology interests against the Roman Catholic Church and conservatives opposed to the research because it involves destroying days-old embryos and cloning.

What’s more, cell research has emerged as a major campaign issue between President Bush and John Kerry, who promises if elected to reverse Bush’s 2001 policy restricting federal funding of such experiments to only those cell lines already in existence.

The measure would authorize the state to sell $3 billion in bonds and then dispense nearly $300 million a year for 10 years to researchers for human embryonic stem cell experiments, including cloning projects intended solely for research purposes. It bans the funding of cloning to create babies.

The amount of money involved far exceeds the $25 million the federal government doled out last year for such research and surpassed even Kerry’s promise to expand funding to $100 million annually.

Many scientists believe stem cells hold vast promise for treating an array of diseases from diabetes to Parkinson’s. Stem cells can potentially grow into any type of human tissue and scientists hope to be able to direct the blank cells to grow into specific cell types needed for transplant.