A 70-year-old linchpin of the American criminal justice system, the exclusionary rule, may be knocked for a loop if government lawyers prevail in a quartet of cases argued this week before the US Supreme Court.
Before a packed gallery, justices here heard evidence and sparred verbally with attorneys. They are trying to decide whether to adopt a so-called ''good faith'' exception to a rule of law - practiced in federal courts since 1914 - which excludes evidence in a criminal proceeding when improper police procedures have been followed.
Many constitutional lawyers say that if the ''good faith'' standard is adopted by the high court, the entire procedure for apprehending and prosecuting those suspected of a crime could be altered. Under the current proposals, argued in the court and strongly backed by the Reagan administration, evidence previously thrown out in a trial would be admissible if it were found that police had not knowingly acted improperly - and, in fact, had acted in ''good faith'' in making an arrest and gathering evidence.
The basic legal question - which has also become a hotly debated political one of late - is whether the exclusionary rule as it now stands is a legal technicality or an effective deterrent to police misconduct and a vital constitutional protection to defendants.
For the court, the issues are complex. And a decision, perhaps a close one with detailed findings, is not expected until later this year. However, the concept of ''good faith'' is one that several justices have already said they are interested in.
Modification of the exclusionary rule is a key agenda item for the administration in its crackdown on crime. It's an issue that will almost certainly surface in President Reagan's reelection campaign. The White House is openly committed to legislative and legal action to tip the scales back from what is seen as overemphasis of the rights of criminals to the detriment of protecting the innocent. Recent attention to strengthening federal laws and court treatment of victims of crime is also evidence of this thrust.
Among arguments offered to the court by state and federal lawyers in support of modifying the exclusionary rule were that it: negates reliable and trustworthy evidence; benefits only the guilty - not the innocent; diminishes public respect for the legal system; discourages proper police procedures along with improper actions; overburdens the courts with costly and time-consuming motions to suppress evidence.
Those opposed to a change in the interpretation of the law, including civil liberties groups and defense lawyers, say it could: seriously set back long-cherished Fourth Amendment rights of the accused; deprive the police of clear guidelines governing their conduct; encourage authorities to ''shop around'' for judges who are lukewarm about defendants' rights; in some cases, promote misconduct by arresting officers who may rationalize that the benefits to society of curtailing criminality outweigh certain protections for the accused.
Two of the cases heard involve criminal convictions that were later reversed by appellate courts because of defective arrest warrants. Another tests the concept of ''inevitable discovery'' - an exception to the exclusionary rule. Under this exception, if the court detemines that some evidence would have been found anyway, it may be admitted, regardless of whether police proceeded properly. The fourth case deals with suppression of evidence.
The Reagan administration is urging that the idea of ''inevitable discovery'' be formally adopted by the court, along with the ''good faith'' exception, to close what it says are legal loopholes for criminals.
Several states already honor the ''good faith'' principle in nonfederal trials. Among them: Arizona, California, Colorado, and Utah.