SACRAMENTO, Calif. — A California prison system that has grown over the past several decades to be the nation’s largest and has been criticized and sued for severe inmate crowding is heading for an historic downsizing.

Some 26,000 guards, janitors, cooks, records clerks and correctional

counselors are being warned they could be laid off, though far fewer will actually lose their jobs.

The move is the result of a new law that shifts lower-level offenders to county jails to save money and reduce state prison crowding. The law was passed to help the state close a $26.6 billion

budget deficit and is expected to trim hundreds of millions of dollars from the state’s largest department.

“It’s quite remarkable that, for the first time since the 1980s, there’s actually a stabilization and eventual shrinking of the prisoner population,” said Joshua Page, a University of Minnesota sociology professor who recently wrote a book on the growth of California’s corrections system.

The downsizing could also strike a significant blow against the politically powerful guards’ union.

The union will be one of the biggest losers as inmates, parolees and the money to house and supervise them flow to the local level, said Frank Zimring, a University of California, Berkeley law professor and expert on the state prisons.

“The largest measure of their political impact was their financial clout,” Zimring said. “Every dime that goes to the local level should be seen as a threat to the guards’ union. Eventually, it will weaken it.”

Department officials are mailing out so many notices to give themselves and their employees flexibility as the system adjusts to a new role handling the most serious offenders, said Robert Downs, chief of the department’s Office of Personnel Services.

Officials said the realignment is projected to reduce the prison population by up to 30,000 inmates over three years and cut in half the number of ex-convicts the state supervises on parole.

The downsizing is a reversal from the past 30 years, during which lawmakers and voters passed a series of tough-on-crime measures that helped spur a boom in the inmate population and prison staffing.

A prison-building surge couldn’t keep up with an inmate population that went from 21,000 in 1976 to more than 159,000 today. The number of employees also swelled from about 12,000 in 1976 to 65,000 today. That’s nearly one of every five state workers.

The union’s growth tracked the inmate surge. It grew from 2,500 members in 1978 to more than 30,000 today, second only to the Service Employees International Union in representing state employees.

The guards’ union became a force that often drove corrections policy while helping its members line up lucrative contracts that made rank-and-file correctional officers the most highly paid of any state.

Former Gov. Gray Davis was criticized for accepting more than $3 million from the union for his 1998 and 2002 campaigns, including $251,000 two months after he agreed to a contract giving them a 34 percent pay raise from 2003-2008.

That was more than twice as high as the average increase for a state employee during that time.

As a result, an entry-level guard earned $45,288 before overtime last year, the most in any state, according to the American Correctional Association. The base salary for a California correctional captain is a top-in-the-nation $98,856.

Davis’ move helped spur a campaign that led to the Democrat’s recall, and the election of Republican Gov. Arnold Schwarzenegger in 2003.

Schwarzenegger’s successor, Gov. Jerry Brown, sought and signed the law that shifts jurisdiction over many criminals to the county level. He argued California had no choice, given its budget deficits and a federal court ruling that prison crowding was leading to increased violence, disease and inmates’ deaths.

The court ordered California to reduce its prison population by some 33,000 inmates over two years, a ruling upheld in May by the U.S. Supreme Court.

California estimates the realignment will save $453 million this fiscal year and nearly $1.5 billion annually by the time the shift is complete in four years.

Other states have reacted to budget shortfalls in recent years by speeding the release of inmates, closing prisons or increasing their use of privately run facilities, according to the National Conference of State Legislatures.

The 26,000 layoff warnings began being sent home Friday to every employee with less than 10 years’ seniority, from janitors to guards. The process is taking days because its mailroom can handle only 6,000 notices a day.

The department estimates that it will eliminate about 3,400 positions from the first wave of warnings. For instance, for roughly each six fewer inmates in the state’s 33 adult prisons, the department will need one less guard, said Downs, the department personnel chief.

Parole officers and others who work outside the prisons will get layoff warnings later, possibly in March, Downs said.

State law and union contracts require the department to give employees 120 days’ warning before actual layoff notices can be issued starting Feb. 29. That gives employees time to retire, find new jobs, or transfer to understaffed prisons.

Actual layoffs will be reduced because the department loses about 50 employees each month to attrition and has kept vacant positions open in anticipation of the cuts.

Officials couldn’t say how many warning letters are going to each job classification.

However, Ryan Sherman, spokesman for the California Correctional Peace Officers Association, said he expects his members to be heavily affected because rank-and-file guards make up about 23,000 of the department’s employees.

His union and the SEIU Local 1000 are among unions that negotiated incentives to encourage their members to move to prison vacancies in other cities as an alternative to losing their jobs.

“Pack up your family and move and pull your kids out of school — not a good thing,” Sherman said. “What’s worse, though, is to be unemployed in this climate.”

Ryan said the union’s first priority is keeping jobs for as many of its members as possible, not maintaining its political clout. “I think that we’ll continue to be a force at the Capitol advocating for our members, no matter how many they are,” Ryan said.

Zimring said the lower prison population may finally let the department concentrate on the rehabilitation. From 1980 to 2000, the department managed growth and then shifted to managing crowding, he said.

“As soon as you get beyond managing crowding and managing growth, you can maybe start talking about doing something for prisoners,” Zimring said.