A round of sincere applause is in order. After seven weeks of thoroughly unpleasant hearings, the Senate Ethics Committee on Tuesday concluded its investigation of the "Keating Five." It was a thankless task, but someone had to do it. Members of the committee did it well.

The harder task of judgment lies ahead: Did their five colleagues violate the Senate's code of ethics? If so, what punishment should be imposed?

The committee faced the same questions last year in the matter of Sen. Dave Durenberger (R-Minn.), but the Durenberger case was easy. The gentleman was a crook. The evidence against him was overwhelming.

In the case of the Keating Five, the problems are far more difficult, and the evidence is in conflict. Here the question involves a line of demarcation. These are the five senators: Cranston of California, Riegle of Michigan, Glenn of Ohio and DeConcini and McCain of Arizona. Collectively they received $1.3 million in campaign contributions from Charles Keating, head of the Lincoln Savings & Loan Association. In one degree or another, all of them sought to assist Keating in his troubles with government regulators.

Did the senators go too far? Did they cross the line that separates proper service to a constituent from improper pressure upon the Federal Home Loan Bank Board? If the line of demarcation were bright and clear, it would be one thing. If the Senate's ethical code were as plain as a criminal code, senators could weigh guilt as a jury weighs guilt.

The trouble is that the line is not fixed. It wavers. Time after time, as the evidence has unfolded, I have been reminded of the uncertain distinction between obscenity, which is prohibited, and pornography, which is not. Justice Potter Stewart, in a famous case, said that he could recognize obscenity when he saw it, "and this isn't it." The same elusive guideline applies to "impropriety."

Griffin Bell discussed the problem before the committee last week. For more than an hour he sat hunched over a microphone, a friendly gargoyle guarding a rain spout, and gave the committee the benefit of his years in public life as federal judge, attorney general and chairman of a presidential commission on ethical standards. He provided the only happy hour in the entire seven weeks.

Bell had come as a character witness for DeConcini, but his testimony dealt primarily with the larger issue. Members of Congress have a duty to assist their constituents. "Nothing wrong with that." Constituents have a right to contribute to a senator's campaign. "Nothing wrong with that either."

But senators have to be careful. "You could get to the point where there would be so much money involved that it would be suspicious."

Sen. Terry Sanford (D-N.C.) got into the discussion: "Our problem is the contention that it doesn't look proper to a reasonable person to do something for someone who has made a contribution. I don't know how we get around that. How do we make certain that it doesn't look improper if indeed it's not?"

Bell offered no easy answer, but he said he had worried from the start of the hearings that the committee might wind up by recommending new rules that would inhibit everybody. Citizens would be reluctant to make campaign contributions, lest the contributions be perceived as bribes, and a senator "might end up saying, you know, I just cannot do anything for you because you made a contribution."

Warren Rudman (R-N.H.), vice chairman of the Ethics Committee, engaged Bell in a reflective colloquy on the functions of government. One function is to shape and to execute policies. In these areas a senator is on safe ground. But other functions are quasi-judicial: Suppose the Federal Trade Commission is weighing an order to cease and desist. Rudman asked what, if anything, a senator might ethically do for a constituent who complained of mistreatment by the bureaucracy in these instances.

"You can inquire," said Bell, "but probably not go much beyond that."

In the case at hand, Cranston and DeConcini clearly went beyond mere inquiry. What of the other three? My impression, after some hours of reading transcripts, is that Riegle, Glenn and McCain probably stayed on the permissible side of the line. My further impression is that no one yet has defined precisely where the line is.