IF YOU ARE the kind of person so strongly opposed to capital punishment that you could never, under any circumstances, impose the penalty, you are known in court as a "Witherspoon excludable." The name derives from a 1968 Supreme Court decision limiting the circumstances in which a prosecutor could reject prospective jurors because of their attitudes about the death penalty. Before the Witherspoon case, jurors who expressed any compunction at all about capital punishment could be rejected by the state. But this resulted, the court said, in juries that were effectively stacked against a defendant and thus more likely to send him to the gas chamber or the electric chair. After 1968, only the hard-core group -- the Witherspoon excludables -- who would automatically vote against the penalty, could be barred from the sentencing phase of the trial.

Most states, however, separate criminal trials into two parts: adjudication and sentencing. While jurors unalterably opposed to capital punishment can be excluded from the latter, there was, until Monday, no clear ruling on whether they could be excluded from the panel determining guilt. The court has now ruled, in a case from Arkansas, that they can. At first blush that may seem reasonable. A juror's belief about the morality of the death penalty is separate from his ability to make a judgment about the guilt or innocence of the accused. But think again and your intuition may lead you in another direction.

Removing Witherspoon excludables from the fact-finding phase of the trial, a number of careful studies show, will leave a group of jurors more likely to believe that a defendant's failure to testify is indicative of his guilt, more hostile to the insanity defense, more mistrustful of defense attorneys and less concerned about the danger of erroneous convictions. Blacks and women would be disproportionately excluded. The research shows that even the very process of making these selections -- creating, lawyers say, a "death-qualified jury" -- focuses attention on the penalty and predisposes those jurors who are finally chosen to believe that the defendant is guilty.

This special prosecutor's advantage comes up, of course, only in capital cases. Ironically, as the three dissenting justices in this week's case point out, it is tolerated only in cases where the offense is most serious and the possible penalty most severe.

A few days ago in this space, we noted that the court seemed to be setting up a series of strict procedural safeguards in capital cases that would make the imposition of the penalty more difficult. Monday's decision to continue the prosecutor's advantage at the fact-finding stage of these most serious cases, is a harsh setback.