A $15 billion bond to renovate California’s aging schools trailed in election returns Wednesday, but the gap was expected to close as several million outstanding votes were counted.

Proposition 13, the only statewide measure on the primary ballot, would provide funds for new construction and repairs at campuses dealing with problems like leaky roofs, old wiring and toxic mold.

The “no” votes led early, but the margin could tighten as ballots roll in from heavily Democratic areas, including Los Angeles County, where the measure has strong support.

Prop. 13 needs a simple majority to pass.

“There are literally millions and millions of ballots still to be counted — and they’re overwhelmingly votes cast by Democrats who support the investment in safe schools,” said Dan Newman with the Yes On 13 campaign. “That’s why with every update, Prop. 13 moves closer to 50 percent.”

Some $9 billion from the measure would go to K-12 schools, with priority given to addressing health and safety concerns such as removing asbestos and eliminating lead from drinking water.

Of that, $5.8 billion would go to updating school facilities, followed by $2.8 billion for new construction and $500 million each for charter schools and facilities for technical education.

Supporters, including Gov. Gavin Newsom, argue the need is crucial. The bond proposal is backed by teachers and firefighter unions, school boards and Democratic state lawmakers.

Opponents say California has a large budget surplus and shouldn’t borrow more money. Taxpayers would owe an estimated $11 billion in interest over the next 35 years as a result of Prop. 13, according to the nonpartisan Legislative Analyst’s Office.

The main opposition comes from the Howard Jarvis Taxpayers Association, which takes issue with a provision that nearly doubles the limit on what a local school district can borrow, from 1.25% to 2% of assessed property value. The group fears that could lead to future tax increases.

Spokeswoman Susan Shelley said Wednesday that with opposition votes leading, “it certainly appears that voters were concerned that this measure would raise local property tax bills.”

The state should fund school facilities itself rather than adding to school districts’ debt, she said.

Many school districts say they do not receive adequate state funding to make repairs and fully modernize their buildings, leading to a Band-Aid approach of periodic repairs and deferred maintenance.

The proposition also would allocate $6 billion for higher education, which last benefited from a statewide bond measure in 2006. The funding would be split evenly, with $2 billion each for community colleges, the California State University system and the University of California system.

According to the Public Policy Institute of California, 70% of California’s 10,000 public schools are 25 years or older, with 10% of them at least 70 years old.

The bond revenue would provide matching funds to school districts that are willing to cover at least 60% of modernization projects and 50% of new construction.

Democratic Assemblyman Patrick O’Donnell of Long Beach, who co-authored the bill that put the bond on the ballot, said now is the time to take advantage of historically low interest rates.

“Bonds are a generally accepted way to finance public facilities that are going to last over the long term,” O’Donnell said. “It’s a cheap loan.”