SACRAMENTO -- California has the nation's largest state prison system, and its most troubled.

Despite a two decade-long building boom that produced 22 new prisons, the state's inmate population remains overflowing at 172,000, more than 70 percent over capacity.

Inside those walls, conditions are so bad that much of the department's operations, including inmate health care, mental health, employee discipline and juvenile justice, have been placed under the authority of court-appointed overseers.

Even Gov. Arnold Schwarzenegger, who made strides toward reform his first two years in office, has been accused of backsliding. Two of his top aides face potential federal court subpoenas over the influence the state's prison guard union has within the administration.

Against that backdrop, Schwarzenegger has called a special session of the Legislature to address wide-ranging prison reform. Lawmakers will begin to consider his proposals after they return Monday from their summer recess, but many are skeptical of Schwarzenegger's motives.

Critics say the Republican governor is merely trying to score political points and keep the influential prison guards union happy so it won't attack him as he faces re-election.

Bills approved in the special session would take effect only about 30 days sooner than if they had been passed during the regular session, while new prisons will take years to build.

"The reality is, it's an election year, the guards' union has $10 million to spend, and both the Republican and Democratic candidates for governor are trying to placate the union," said Dan Macallair, a prison reform advocate with the San Francisco-based Center on Juvenile and Criminal Justice.

Schwarzenegger's Democratic opponent, state Treasurer Phil Angelides, has pledged to call a state of emergency in the state corrections system if he is elected in November and has called for his own aggressive prison-building program.

The prison guards union stands to gain thousands of new members under both plans.

The centerpiece of Schwarzenegger's special session is a proposal to spend $6 billion to build two prisons and a variety of other inmate lockups that will create space for rehabilitation and training programs.

The governor's aides say new prisons are needed to solve a number of the system's most pressing problems. They will relieve overcrowding, create space to provide rehabilitation, job-training and drug-treatment programs to inmates about to be paroled, and help handle a rising tide of prisoners.

The prison population is expected to hit almost 200,000 within 15 years, double the current capacity.

"The governor has no choice but to try to reform the system because if he doesn't get his arms around the crowding, a judge will come in and order early release," said state Assemblyman Todd Spitzer of Orange, who is leading the prison negotiations for the Assembly's Republican caucus.

Critics of the governor's reform plan say the special session will be meaningless if it fails to address some of the key causes of the inmate population boom -- sentencing laws and a parole system that require convicts to be locked away in state prisons for nonviolent offenses.

Even the California Correctional Peace Officers Association, the union representing prison guards, suggests the state create a commission to independently review sentencing laws and recommend changes.

"We want to be tough on crime ... and we forget that these sentences come at a cost," said Assembly Speaker Fabian Nunez, D-Los Angeles.

The prison population has been fed by the state's growing population and tough sentencing laws, led by the "three-strikes" initiative passed by voters in 1994. That law puts three-time offenders behind bars for life.

Voters rejected a softening of the law in 2004 after a last-minute opposition campaign led by Schwarzenegger, who has ruled out any changes since then.

Yet a study from the Center on Evidence-Based Corrections at the University of California, Irvine shows the state's incarceration rate is not high by U.S. standards. The study, made public this summer, also found that 70 percent of California's prison population boom over the last decade was driven by violent crime, not those convicted of drug or property crimes who could be housed outside the prison system.

The governor's office has used the study to support its building program.

Schwarzenegger called a special session after legislators balked at proposals he outlined in January to build new prisons and move nonviolent women to rehabilitation programs, said Adam Mendelsohn, Schwarzenegger's communications director.

"The entire purpose of the special session is to address the overcrowding problem -- by addressing the highest recidivism rate in the country and by creating more space," Mendelsohn said. "All the experts will tell you, without space you cannot have rehabilitation."

About 70 percent of former inmates return to prison within three years after they are paroled, nearly twice the national average.

"We're doing nothing to assist those inmates to be better when they come out of the prison system," Acting Corrections Secretary James Tilton said. "Their chances of being successful go way down."

Building more prisons and local lockups will free space for rehabilitation programs that could help parolees transition into society after their release, he said. Fewer parolees committing crimes means fewer being sent back to state prison.

Other options also would reduce the need for new prisons, said Joan Petersilia, a criminology professor at the University of California, Irvine, who prepared the incarceration study.

She supports Schwarzenegger's plan to move 4,500 female prisoners into smaller community-based centers and said 3,000 elderly inmates and 18,000 others who are in prison for technical violations of their paroles could be sent to alternative programs. Such moves would quickly ease prison crowding, she said.

"I'm not ruling out new prisons at all. I just don't think we've demonstrated the need," Petersilia said. "I'd like less hot air and more hard facts."

Underlying the debate over new prisons will be the cost.

Corrections-related expenses consume an ever-greater share of taxpayer money, reaching nearly 9 percent of the state's annual budget -- or $8.7 billion in the current fiscal year. Further, the various overseers appointed by federal and state judges have wide latitude to implement their own changes, reforms that are expected to cost billions of dollars more in the coming years.

"This issue is as big as the energy crisis in the Davis administration, I believe, and has costs that will be as big," said state Sen. Jackie Speier, D-Daly City, who has led hearings into wasteful spending in the corrections department.