THE MOST controversial issue in the current Senate ethics debate is how much senators should limit their outside earnings, primarily speaking fees. This focus is encouraging. It suggests that broad financial disclosure rules, a ban on unregulated "slush funds" and other tough provisions in the proposed code have become much less controversial. It also shows that nowadays senators, compared to House members, are less inclined to practice law part-time, collect large fees as corporate directors or carry on other business activities that raise conflict-of-interest questions.

The senatorial specially is speech-making. The lecture circuit can be lucrative; in 1973, various groups paid senators more than $1 million in honoraria. In 1974, 22 senators earned more than $15,000 each that way. Because this raised questions of seemliness, Congress then limited honoraria to $15,000 per year and $1,000 per appearance. The ceilings were raised to $25,000 and $2,000 last year. The pending Senate code would bring the earned-income limit down to 15 per cent of the official salary, or $8,625 - the figure already adopted by the House.

Sen. Edmund Muskie (D-Maine) and others argue that the lower ceiling would be unfair to senators who lack independent wealth. Either the $25,000 limit should be retained, they say, or unearned income should also be limited. Sen. Muskie's position is especially credible because he has spent 30 scandal-free years in public offices, including posts with paltry salaries, and has taken pains to avoid investments that might involve the slightest conflicts.

Nevertheless, this argument has some flaws. Earned and unearned income are not alike. Stock dividends, for instance, flow at the same rate to all investors. Speaking fees do not. Moreover, honorarian are a direct result of one's official power and prestige, and often come from groups with a large interest in fostering senatorial goodwill. Majority leader Robert Byrd (D-W. Va.), for example, is a pretty fair fiddle-player. Yet as he said the other day, "I don't think any group of citizens would pay me $2,000 to play my fiddle for 15 minutes if I were a meatcutter or working in a shipyard or practicing law."

Of course, unearned income is not always conflict-free. A lawmaker's actions can affect or be influenced by his investments, too, though the connections are usually more remote. While disclosure solves many problems, a super-strict ethics code would require legislators to put their holdings in blind trusts, as top executive-branch officials must do. The purpose of blind trusts, however, is not to limit income but to separate personal gain and official activities. Since honoraria cannot be made "blind," a consistent application of the separation principle would include barring senators from getting any fees at all from groups with any interest in legislation.

That is not what Sen. Muskie and his allies have in mind. Their greateset concern seems to be preserving honoraria as an income supplement. But the point of the pay raise to $57,500 was to make official salaries more adequate and reduce the need or justification for getting payments from interest groups. Many people, in fact, consider the pay raise defensible only in exchange for greater congressional self-discipline. Thus, lower limits on outside earnings, while not as vital as full disclosure and other rules, are timely and reasonable.