Because lawyers will always be lawyers, it was predictable that lawyers for Dennis DeConcini, Alan Cranston and Don Riegle would mount a familiar defense: The lawyers contend that the three senators are being denied "due process of law." It isn't so, but the charge may properly be examined.

As everyone knows the three are now the objects of a hearing before the Senate Ethics Committee. They have been accused of wrongfully intervening in the case of Charles H. Keating, head of the now-failed Lincoln Savings and Loan Association. Two other senators, John McCain of Arizona and John Glenn of Ohio, have been pretty well exonerated.DeConcini, Cranston and Riegle make these arguments: This is not a "hearing" or an "investigation." The proceeding has all the trappings of a trial. The committee's special counsel, Robert S. Bennett, is a prosecutor. The committee functions as judge and jury. Counsel offer motions that may be sustained or overruled. Evidence is admitted. In such a quasi-criminal setting, the defendants are entitled to the rights that constitute due process.

The principal contention is that the Senate's standard of ethical conduct is so loosely phrased that it should be held "void by reason of vagueness." Under the rule, no senator may accept "favors or benefits under circumstances which might be construed by reasonable persons as influencing the performance of their governmental duties."

Keating and his associates contributed about $1.3 million to the re-election campaigns of the five senators. The five are accused of intervening actively on Keating's behalf when federal regulators were zeroing in upon the Lincoln operation. Their defense is that they did no more than other senators do.

Is the standard so vague that senators cannot know when they have overstepped it? Lawyers contend that no senator may know exactly what is meant by "circumstances," or by "might be construed" or by "reasonable persons."

Analogies may be found in other areas of public and professional life. Consider Article 133 of the Uniform Code of Military Justice. It proscribes "conduct unbecoming an officer and a gentleman."

Over the years, Article 133 repeatedly has been attacked as void for vagueness. It repeatedly has been upheld. In the case of Parker vs. Levy, the Supreme Court held that "longstanding customs and usages of the services impart accepted meaning to the seemingly imprecise standards." Would this not be true of the Senate?

Bennett, the committee counsel, cited other examples on Nov. 19. Federal judges must conduct themselves "in a manner that promotes public confidence in the integrity and impartiality of the judiciary." Lawyers are governed by a written code that requires them to avoid "even the appearance of impropriety."

It is no problem in criminal law to define auto theft, bank robbery, kidnapping or fraud. But in the subjective areas of senatorial ethics specificity is an unattainable objective. One is reminded of Justice Potter Stewart's definition of obscenity: "I know it when I see it, and the motion picture involved in this case is not that."

That is about where we are in the case of the "Keating Five" a case now reduced to the "Keating Three." It is the thankless task of the Ethics Committee to decide if the senators knew that their relationship with Charles Keating created an appearance of serious impropriety.

In the same way that judges recognize obscenity, senators should recognize impropriety when they see it. There's enough of it around.