In a move that would broadly expand public education, California Democrats on Tuesday proposed creating universal preschool for 4-year-olds, to try to enroll more children from immigrant and low-income families.

The program would be voluntary, just as kindergarten is in California. It would phase in over five years beginning in the 2015-16 school year and could cost the state $1 billion annually when fully implemented — adding an estimated 350,000 children to public schools.

Proponents argue that those costs would be offset by savings later on. They point to studies showing that preschool prepares children for school, resulting in fewer being held back to repeat a year, lowered numbers in costly special-education classes, and fewer students failing in school and ending up in prison.