THE FIRST decision the Senate Ethics Committee must make in judging the Keating Five is which lens to use. The five accused senators and their defense attorneys will be urging that it use a close-up. They will want to subdivide the whole affair, to examine each particular act of each senator in isolation and ask which specific Senate rule it violated.

That's fair enough, and precisely what the capable attorneys are being paid for. But the ethics committee has a responsibility to use a broader lens as well. It is not a court, or just a court. It represents the Senate, and is being asked not just to find whether certain rules were broken but to declare the boundaries of acceptable behavior.

Here are five senators -- Alan Cranston, then Democratic whip, the No. 2 position in the party hierarchy and the second-ranking Democrat on the Banking Committee; Donald Riegle, Banking Committee chairman; Dennis DeConcini and John McCain of Arizona; and John Glenn of Ohio -- all of whom accepted large political and in some cases other contributions from Arizona-based savings and loan operator Charles Keating. As a group and in some cases individually, the senators then intervened in his behalf, and not in the usual kind of transaction. The issue here was not a new contract for federal pencils, but the enforcement of regulations meant to ensure the integrity of the financial system.

The regulators were sniffing at Mr. Keating's Lincoln Savings and Loan Association of California. He wanted them to back off; their distinct impression was that that was what the senators wanted of them too. Lincoln eventually collapsed, and may cost the taxpayers $2 billion.

Special counsel Robert Bennett has found that Sens. Cranston and DeConcini played major roles in the affair, Sen. Riegle a larger role than he has acknowledged, and Sens. McCain and Glenn lesser roles. That is the first of many such distinctions that will be made; the committee will have to decide which are justified. The argument will also be made -- it has been already -- that this was only normal Senate casework on a larger scale. If this was wrong -- the thinking continues -- which senator can ever again safely intervene with a federal agency on behalf of a constituent who was unwise enough to have contributed to his campaign? If it will not rely on legalistic distinctions, the ethics panel will be asked to make none at all.

What it needs to do instead is reach, at least in part, a moral judgment. What happened here was not casework. The senators are accused of having crossed a pretty basic line. If jeopardizing the financial system by bulldozing its appointed regulators on behalf of a big political contributor is not a violation of Senate rules, you have to wonder what is. The Senate is on trial here, not just five senators.