The Supreme Court decided by one vote yesterday that a man convicted of a "senseless and brutal" child murder must have a new trial because Iowa detectives had denied his constitutional right to a fair trial.

The 5-to-4 decision turned back pleas by 22 states, including Maryland and Virginia, for dilution or abandonment of the Miranda rule and other broad constitutional protections that the court under Chief Justice Earl Warren had provided for criminal defendants.

The justices wrote seven separate opinions. Their sometimes harsh and impassioned language suggested that the case was unusually divisive, partly because of its sensitivity: the victim was a 10-year-old girl who was kidnaped on Christmas Eve, 1968, raped and suffocated.

Moreover, Chief Justice Warren E. Burger, in a rare action, read aloud a 15-page dissent attacking the decision as one that "ought to be intolerable in any society which purports to call itself an organized society." He then went beyond this to say from the bench that the court someday might restore "rationality" with "only one convert."

Burger apparently referred to Justice Lewis F. Powell Jr., who may have cast the swing vote and who was the only appointee of President Nixon in the majority.

Powell, in turn, wrote that Burger "disregards" and misperceives key facts bearing on the critical issue: had the accused man, former mental patient Robert Anthony Williams, voluntarily waived his right to counsel under the Sixth and 14th amendments?

On Dec. 24, 1968, the family of the victim, Pamela Powers, brought her to the YMCA in Des Moines to watch a wrestling tournament. She left her seat to go to a washroom but did not return. A while later Williams, an escapee from a Missouri mental institution who had moved into the "Y," went through its lobby carrying a large bundle.

He asked a 14-year-old boy to open the street door of the building and then the door of his car. Williams put the bundle on the front seat. The boy later testified that he saw two legs in it.

The abandoned car was found Christmas day, in Davenport, Iowa, 160 miles east of Des Moines. The girl's whereabouts were unknown. Authorities in Des Moines then issued a warrant charging Williams with abduction.

The next morning, lawyer Henry T. McKnight of Des Moines reported to police there that Williams had phoned him and that he had advised him to surrender in Davenport.

Williams did so. In compliance with the Warren court's Miranda V. Arizona decision of 1966, officers then - and several times later - informed him of his right to remain silent and to get an attorney's advice. They warned him that any statements be made could be used against him.The Miranda decision makes inadmissible confessions from suspects not advised of these rights.

McKnight made an agreement with the Des Moines police chief and Detective Capt. Cletus Leaming that Leaming and another detective would drive to Davenport, pick up Williams, return directly and not question him during the trip.

In the officers' presence, McKnight told Williams of the arrangement and instructed him to say nothing about Pamela Powers until after returning to Des Moines and cousulting him.

In Davenport, meanwhile, Williams was arraigned and jailed on the abduction warrant. Another lawyer, Thomas Kelly, reminded him to stay silent. When Captain Leaming arrived, Kelly also reminded not to interrogate Williams. When Kelly asked to ride with him and Williams back to Des Moines, Leaming said no.

Soon after leaving Davenport, Leaming, who knew Williams was deeply religious and who addressed him as "Reverend," discussed religion and made what came to be known as the "Christian burial speech." Leaming said that several inches of predicted snow would conceal Pamela's body so that even Williams, "the only person that knows" where it is, "may be unable to find it."

Leaming suggested that they stop to look for the body to assure "a Christian burial." he claimed, he knew it to be in the area of Mitchellville, a town along the way. "I don't want to discuss it further," Leaming said. "Just think about it as we're riding down the road."

Finally, near Mitchellville, Williams offered to, and did, take Leaming and another detective to the body.

At his trial, the prosecution acknowledged tha the burial speech was tantamount to interrogation. Leaming, asked if he had tried "to get all the information you could before Williams got back to McKnight," replied, "Yes, sir."

A jury convicted Williams. The Iowa Supreme Court affirmed the conviction 5 to 4. But a federal judge, a divided Eighth U.S. Circuit Court of Appeals and the U.S. Supreme Court all held the conviction invalid because, they said, Williams was denied the right to counsel in an interrogation after a judicial proceeding formally had started. The lower federal courts also cited the Miranda ruling and other ground that the high court said it "saw no need to review . . ."

For the court majority, Justice Potter Stewart said the violation of the right to counsel "cannot be condoned." The predictability of pressures "when the crime is murder and the victim a small child" makes "imperative a resolute loyalty" to the Constitution's guarantees "to us all," he said.

Stewart relied on a 1938 decision that the right to counsel is in force unless "intentionally relinquished." Chief Justice Burger said Williams had "made a valid waiver" of the right.

Burger, with dissenting Justice Harry A. Blackmun agreeing, said that what Stewart called an "interrogation" was "a statement" by Leming. Justice Powell termed it "skillful and effective . . . interrogation and psychological coercion."

Dissenting Justice Byron R. White, joined by Justices Blackmun and William H. Rehnquist, said the police "did nothing 'wrong'" and denounced the majority opinion - which he said was based on "nothing in the Constitution" - as "utterly senseless."

Justice Thurgood Marshall said he doubted that Williams - facing a new trial - will ever go free despite "blood-curdling cries" by the dissenters, but said that if he does it will be because of Leaming's "intentional police misconduct."

Justice John Paul Stevens said the state had captured "a potentially dangerous person" by making a promise that it "cannot be permitted to dishonor . . ."

Marshall and Stevens joined the majority ruling.