THE CHARGES against Sen. Dave Durenberger (R-Minn.) bespeak an institutional as well as an individual failing. For fear of offending the voters and to pose as small-d democrats, senators have kept their official salaries artificially low over the years. At the same time, they have allowed the interest groups, whose legislative fortunes they control, to supplement their incomes in the form of honoraria or supposed speaking fees of up to $2,000 an occasion and 40 percent of salary each year.

For Sen. Durenberger, not even this was enough. Instead of giving his excess speaking fees to charity as the rules require, he laundered them through a book publishing venture, converting the payments into promotional fees on which, unlike honoraria, no limits are imposed. He also took lodging expenses for staying in a Minneapolis condominium he owned.

The six-member Senate Ethics Committee, three from each party, found unanimously that his conduct was "unethical," "reprehensible," violated the rules and had "brought the Senate into dishonor and disrepute." The panel recommended that he be denounced by the Senate, a form of censure, and ordered to make more than $124,000 in restitution. The case will also be referred to the Justice Department to decide if there were violations of the law and to the Senate Republican caucus to judge whether he should be stripped of committee seats or his 12 years of seniority.

Mr. Durenberger, who is not up for reelection until 1994, is not seriously contesting the charg/es against him. He said he made "serious mistakes" and is sorry. But the Senate has more to do than just denounce him. There are, for one thing, six more cases before the Ethics Committee, including the celebrated five senators from both parties who received large political contributions from failed California savings and loan operator Charles Keating, and who then intervened with federal regulators in his behalf.

These cases aside, the Senate must look to its rules. Last year the House finally and forthrightly did the right thing on compensation, voting to raise its pay and give up honoraria. The Senate weakly refused to do the same. The issue will recur this year, and the Senate should follow the House's lead. No more of the gray world of dual paymasters in which Mr. Durenberger operated. Majority leader George Mitchell (D-Maine) should likewise bring to a vote the campaign reform legislation on which he has been dickering too long with reluctant members of his own party as well as Republicans. The Durenberger vote is not how senators can show they support clean government. They'll show that, or not, by how they vote on the subsequent measures that affect themselves.