ASHINGTON, March 8 — The Supreme Court overturned an assault conviction from Washington State today as it broadened and sharpened a defendant's constitutional right to confront witnesses against him.

In a case that has been closely watched by defense lawyers and prosecutors, the justices unanimously set aside the conviction of Michael D. Crawford of Olympia, who has been serving a 14-year sentence for stabbing a man who he apparently believed had tried to rape his wife.

The stabbing took place on Aug. 5, 1999, in the apartment of the victim, Kenneth Lee, in the presence of the defendant's wife, Sylvia. Mr. Crawford told the police that he had acted in self-defense. The case might have played out as a commonplace incident, ugly but routine, except that Mr. Crawford invoked the marital privilege under Washington State law to prevent his wife from testifying against him.

At trial, prosecutors introduced a tape recording of the statement that Mrs. Crawford had made under police questioning. They did so because there were discrepancies between the accounts of husband and wife, and prosecutors thought the wife's statement would undermine her husband's claim of self-defense.

In reversing the conviction today, the Supreme Court, in an opinion by Justice Antonin Scalia, cited the familiar phrase in the Sixth Amendment that in all criminal prosecutions the defendant shall enjoy the right "to be confronted with the witnesses against him."

"The right to confront one's accusers is a concept that dates back to Roman times," Justice Scalia wrote.

That general principle of the Sixth Amendment has not been absolute in practice. A 1980 Supreme Court decision spelled out certain instances in which the testimony of an unavailable witness may be allowed at trial, among them when the trial judge deems the witness particularly reliable for various reasons.

Today's ruling effectively erased the 1980 decision and severely curtailed, while not eliminating, prosecutors' ability to use the accounts of witnesses who cannot be cross-examined during trial. The American Civil Liberties Union and the National Association of Criminal Defense Lawyers had filed briefs urging the conclusion that the court reached today.

In voiding the verdict and sending the case back to the lower courts, possibly for retrial, Justice Scalia held that the Sixth Amendment commands "not that evidence be reliable, but that reliability be assessed in a particular manner: by testing in the crucible of cross-examination."

Alluding to the 1980 decision, Justice Scalia wrote: "Dispensing with confrontation because testimony is obviously reliable is akin to dispensing with jury trial because a defendant is obviously guilty. This is not what the Sixth Amendment prescribes."

The ramifications of today's ruling may not become clear immediately, but they could affect a number of previous convictions across the country. The testimony of a witness who cannot be cross-examined during trial may still be allowed, but only if the defense has had a chance to cross-examine him or her at an earlier stage in the case.

Chief Justice William H. Rehnquist filed a separate opinion, joined by Justice Sandra Day O'Connor, in which he agreed that Mr. Crawford's conviction should be set aside. But the chief justice said Justice Scalia had swept aside the court's 1980 holding without sufficiently defining for "thousands of federal prosecutors and the tens of thousands of state prosecutors" exactly what rules they can apply.

"They need them now, not months or years from now," the chief justice wrote.

Today's ruling in Crawford v. Washington, No. 02-9410, reversed a ruling by the Washington State Supreme Court. The state high court had reinstated the conviction after it had been set aside by a state appellate court.