Washington — Legislation that would radically narrow the insanity defense in criminal cases may soon be enacted by Congress in the wake of the jury's decision that John W. Hinckley Jr. was not guilty by reason of insanity in the attempted assassination of President Reagan.
''There is growing support for a quick vote on it,'' says a spokesman for Sen. Strom Thurmond (R) of South Carolina, chairman of the Senate Judiciary Committee and sponsor of an ''insanity defense provision'' in the violent crime and drug enforcement improvements act of 1982. The spokesman says the bill, which has 52 cosponsors including Sen. Edward M. Kennedy (D) of Massachusetts, is expected to pass easily.
The bill removes the traditional definition of the insanity defense. Senator Thurmond's spokesman says: ''Forget about (the argument) of irresistible impulse and whether the person knew right from wrong. The question is only did he have the mental state necessary to commit the crime.'' If he did - if he knew he was committing the crime - then, according to the proposed bill, he is guilty.
''Guilty but insane,'' the new verdict, would require that the defendant be kept in an institution as long as the normal imprisonment term for the crime.
''If you talk to the ACLU, they say it absolutely abolishes the insanity rule ,'' says a Senate Judiciary Committee spokesman. (The American Civil Liberties Union was unable to comment on the bill at this writing.) He denies that it abolishes the insanity plea but says it is ''a very narrowing definition,'' and explained that ''what we're talking about is a situation in which the insanity plea can be used only when someone is so mentally ill they don't know they're taking a life, or that it's the life of another human being . . . where someone is operating under the delusion that it's an animal (as in the 'Son of Sam' case) or the devil urging them, someone who does not comprehend that he's committing this crime.''
The legislation would call for mandatory commitment of anyone found guilty but insane - closing a loophole that now exists in most states. Only the District of Columbia, where Mr. Hinckley was tried, now requires commitment for those found insane in a criminal trial. Hinckley was found ''not guilty by reason on insanity'' June 21 on 13 counts, including wounding President Reagan, presidential press secretary James Brady, District of Columbia police officer Thomas K. Delahanty, and US Secret Service agent Timothy McCarthy.
Senate Republican leader Howard H. Baker Jr. of Tennessee said Tuesday that he backs the Senate bill and that the insanity plea has a place in federal law, but that the burden of proof should be changed so that it is up to the defendant to prove he is insane, not on the government to prove he is sane.
Meanwhile, members of Congress and the administration have spoken out strongly on the verdict and the insanity plea. Attorney General William French Smith has just called for a reform of the insanity defense in which the individual would be acquitted only if a jury found he did not know right from wrong.
''There must be an end to the doctrine that allows so many persons to commit crimes of violence, to use confusing procedures to their own advantage, and then to have the door opened for them to return to the society which they victimized, '' Mr. Smith said.
In Washington there has been surprise and in some cases shock on the verdict, which in the 50 states would have set Mr. Hinckley free to walk the streets again. He will now be committed for 50 days to a local hospital for the mentally ill. Then the court must hold a hearing to determine if he is still mentally ill or if he should be released because he is not dangerous to himself or the community. US District Court Judge Barrington D. Parker has scheduled a July 12 ''sentencing hearing'' for Hinckley in the trial.
Dr. Alan Stone, a Harvard law professor and expert on the insanity plea, says of the jury: ''They are to be admired. They did what the law told them to do. The problem is not with the jury, it's with the law . . . . They don't need to abolish the insanity defense. The real issue is the protection of society. . . .
One informed legal source said Hinckley might be evaluated as dangerous to himself or society but not insane, in which case he might not be released.