FIRING SQUADS are used in two states and hangmen in four. Gas chambers are the preferred method in seven jurisdictions, electric chairs in 14 and lethal injections are state of the art in 16. Since 1972, when the Supreme Court invalidated every death-penalty law in the country, statutes have been rewritten in most states. Executions are proceeding apace -- there have been 51 since 1977 -- but so far Congress has not passed a broad capital punishment law that is constitutionally acceptable to the courts. Now, the pressure is on again; the Senate Judiciary Committee has ordered a bill reported, and it may be ready for floor action as early as next week.

The bill is similar to one passed by the Senate in 1984. It would allow executions for federal crimes involving the death of a person, for treason, for aggravated espionage and, in some circumstances, for attempted assassination of the president. A two-tier trial would be held, the first to determine guilt or innocence and the second to determine punishment. The jury would consider both the aggravating and the mitigating factors, and before pronouncing the death sentence would have to find that the former outweighed the latter. Supporters believe that these procedures and limitations will safeguard the statute from constitutional challenge, and they may be right. But even if the courts found such a law to be consistent with the Eighth Amendment, it would still be repugnant, degrading and unwise.

In one critical respect, the Senate bill is an improvement over versions that were considered in the past: this bill would prohibit executions for crimes committed before a defendant's 18th birthday. This small exclusion hardly seems spectacularly merciful, but in fact only six states have similar provisions in their capital punishment laws. In one state, anyone who is over 10 when he commits a capital crime can be executed. There are dozens of young men on death rows across the country for murders they committed as minors, and one, Charles Rumbaugh, was executed in Texas last September. Bad as it is, the proposed new federal law would not allow that kind of punishment for youthful crime.

The attempted assassination of President Reagan in 1981 and the flood of recent espionage cases have increased public support for restoration of the death penalty. It will be easy for lawmakers to respond to pressure and to take a hard line. But senators who question the efficacy, the justice and, yes, the morality of the act must continue to resist.