Supreme Court Justice Thurgood Marshall said yesterday that recent high court decisions and inexperienced trial lawyers often deprive those facing the death penalty of a fair chance to defend against their executions.

Defendants facing the death penalty are "caught in an increasingly pernicious vise grip" -- squeezed on the one hand by Supreme Court limitations on federal court review of state trials and on the other by untrained volunteer or court-appointed trial lawyers, Marshall said.

By the time lawyers skilled in the intricacies of death penalty law enter the case, they are "too late to help a convicted defendant," Marshall said, in part because re- cent decisions have speeded the process and made it impossible to undo errors made at the initial trial level.

Marshall and Justice William J. Brennan Jr. are the only justices to oppose the death penalty in all instances.

More than 1,500 inmates nationwide are on death row. There have been 47 executions since the court reinstated the death penalty in 1976.

The pace has accelerated recently, with 15 executions so far this year.

Marshall's remarks came in a speech prepared for a conference of federal judges in Hershey, Pa., and released here.

Inexperienced defense lawyers, he said, "though acting in good faith, inevitably make very serious mistakes," which cannot be corrected on appeal.

"The federal reports are filled with stories of counsel who presented no evidence in mitigation of their client's sentences because they did not know what to offer or how to offer it," or had not even read the state's sentencing law, Marshall said.

"I kid you not," he said, "precisely that has happened time and again."

He criticized the Supreme Court majority for not recognizing that the constitutional right to effective legal assistance "must encompass a right to counsel familiar with death penalty jurisprudence."

"Whatever your views about the death penalty," he said, "we simply cannot accept this state of affairs."

Marshall said he continues to oppose the death penalty, "but as long as our nation permits executions, lawyers, judges and public officials have a duty to assure that people who face the ultimate sentence" have a chance "to present their best case."

"The bar must focus on improving the quality of trial counsel," he said, "and must find resources to establish training and assistance" for local lawyers appointed to handle death penalty cases.

"Contrary to popular perceptions," Marshall said, "all capital defendants have not spent years filing frivolous claims" in federal courts.

Many of them, he said, have not filed any federal court claims when their execution dates are set.

Those dates generally are set one month before the execution, he continued, and "once the date is set, the race is on."

That is often when veteran lawyers step in to file last-minute appeals.

"When the process speeds up," he said, "the opportunity for deliberation, consideration and rebuttal vanishes," he said.

Marshall said that until the Supreme Court guarantees those facing the death penalty a fair chance in court, "others must work within the existing system to provide that opportunity."

Marshall said states could take steps to avoid such a last-minute rush by setting execution dates farther in advance.

He noted with approval a state- financed public defender's office in Florida recently established to focus only on death penalty cases.

Where states have not set up such offices, "members of the private bar must attempt to fill the same role," he said.

Meanwhile, the Texas Civil Liberties Union said yesterday that it cannot halt two executions scheduled for next week because the two condemned killers have not authorized anyone to file appeals on their behalf.

Charles Rumbaugh, 28, convicted for the 1975 murder of a jeweler during a robbery, is scheduled to be executed by injection just after midnight Wednesday.

John Henry Selvage, 35, convicted in the 1979 murder of an off-duty police officer during a jewelry store robbery, is scheduled to die just after midnight Thursday.