For more than two decades, the gas chamber at San Quentin prison has been idle while California built up one of the nation's largest Death Row populations.

For those 231 prisoners, all of them men, the countdown has begun."The cases are moving along now," said Gary Mullen, executive director of the California District Attorneys Association, the state's foremost prosecutors' organization. "We expect that in the near future, probably before the end of the year, there will be a death penalty case that will be cleared all the way through the state Supreme Court."

"There has been a dramatic change in the (state) court," adds John Meehan, district attorney of Alameda County and one of the state's foremost death penalty experts. "An execution could occur by the end of this year or the beginning of next year."

Throughout the nation, 100 people have been executed since the U.S. Supreme Court allowed states to restore the death penalty in 1976. So far, only 12 of the 37 states with capital punishment laws have resumed executions. Of the 25 states which have not, California has the largest death-row population.

This year, the California Supreme Court reversed one of its most important earlier rulings that impeded executions, declaring that a killer can be put to death even if there was no intent to murder.

The ruling marked a sharp departure from the philosophy of the previous Supreme Court, dominated for a decade by the liberal appointees of former Gov. Jerry Brown. His chief justice, Rose Bird, was removed from office by voters in 1986 along with two liberal colleagues.

In one year, the new court under Chief Justice Malcolm Lucas has affirmed more than a dozen death sentences, compared to four death sentences affirmed during the 10-year tenure of the Bird court.

The court now is controlled by a 5-2 majority appointed by Republican Gov. George Deukmejian, who as a state senator sponsored the 1977 death penalty act.

Under California law, all death sentences are automatically reviewed by the state Supreme Court. If the state high court affirms the decision, the death sentence is appealed directly to the U.S. Supreme Court, followed by a second round of challenges, raising issues not addressed previously and often focusing on the competency of the defendant's original attorney. The litigation commonly takes years.

"There has been a dramatic turnaround," said Mullen. "I think the state Supreme Court has become more mainstream - 80 percent of Californians support the death penalty, according to polls."

California's Legislature moved quickly to restore capital punishment following the U.S. Supreme Court's 1976 ruling that permitted executions to resume.

A narrow death penalty bill, sponsored by Deukmejian, was approved by the Legislature in 1977 and vetoed by Brown. The Legislature, for the first time, overrode Brown's veto.

In November 1978, California voters approved a death penalty initiative extending capital punishment to accomplices and other types of murders not covered by Deukmejian's legislation. Most of the controversial state Supreme Court rulings dealing with the penalty have focused on the meaning and constitutionality of the initiative.

Depending upon the course of their respective court battles, the next to die will probably be one of three killers: Robert Alton Harris of San Diego, Stevie Lamar Fields of Los Angeles or Earl Lloyd Jackson of Long Beach.

"They are regarded as the furthest along," said Bob Gore, a spokesman for the state Corrections Department. "As to which one it will be, right now we can't say."