SACRAMENTO, Calif. (AP) — California lawmakers on Monday approved a plan to borrow $6 billion from a state savings account to pay down massive debt in the nation's largest public-employee retirement program.

The Senate sent the measure to Gov. Jerry Brown despite concerns from some lawmakers that it could be risky to borrow money and invest it through the California Public Employees' Retirement System.

CalPERS' $325 billion investment fund, the largest public-employee pension fund in the country, has enough money to cover only about two-thirds of the benefits promised to retired public employees. The CalPERS board has steadily increased mandatory contribution rates for the state, cities, counties and school districts to make up for the shortfall. The state's share of the unfunded liability is estimated to be $59 billion, with its minimum payment forecast to increase from $5.8 billion this year to $11.2 billion in 2031.

Brown in May proposed borrowing money to pay down that debt, saying the state can lend from its own short-term investment account and earn a higher return through the more aggressive investments at CalPERS. He said the proposal would save $11 billion over 20 years through lower annual pension costs.

Money borrowed from the short-term account, known as the Surplus Money Investment Fund, would be repaid with interest using money from Proposition 2, which was approved by voters in 2014 and requires the state to spend a portion of revenue paying down debts.

Republican Sen. John Moorlach of Costa Mesa said he likes the idea of prepaying retirement costs and even joined Democratic Treasurer John Chiang in writing a newspaper editorial supporting the plan. But he voted against it Monday, citing Brown's repeated warnings that a recession is inevitable.

"If that's the case, we will have put $6 billion into the retirement system at the most inopportune time," Moorlach said. "That is my concern, that we invest at the top of the market."

CalPERS has struggled to hit its own investment target, which is phasing down to 7 percent per year. Last year, the fund earned 0.61 percent, down from 2.4 percent the prior year.

Democratic Sen. Steve Glazer of Orinda said the idea may have merit but lawmakers should wait until it's been more thoroughly studied.

"A few months of analysis in a public setting so everybody can hear, not in a back room, can only be a healthy thing for us to ensure this is the right course to take," Glazer said.

H.D. Palmer, a spokesman for Brown's Finance Department, said the proposal was carefully developed with insight from lawmakers and Chiang, who has a legal obligation to responsibly oversee the state's investments.

"We've fully disclosed all of the potential risks and inherent benefits it entails, and we are convinced that it's a sound and prudent step to take," Palmer said.

The measure was approved in a 24-13 vote with support mostly from Democrats.