Sen. Edmund S. Muskie (D-Maine), with never a hint of personal scandal around his 30 years in elective office, the last 18 as senator, is about to become the leader in the fight against earned income limitations in the newly's drafted Senate ethics code.

The measure is expected to reach the Senate floor Wednesday.

"I feel like protesting," the 62-year-old Muskie said the other day in a feisty tone.

Muskie is too crafty a politician to take the matter head on, instead, he plans to offer an amendment that would expand the limitation to invesment as well as earned income.

That move would focus attention on the whole income question and could very well force a uniform policy for all outside Senate income, either disclosure or limitation.

As drafted by a special committee on official conduct, chaired by Sen. Gaylord Nelson (D-Wis), the code would limit the amount of earned income a senator could make in any year to 15 per cent of his salary.

With salaries at $57,500, a senator could earn an additional $8.625 a year.

However, a senator with investment income could take in an unlimited amount. The code would only require that he make a full disclosure.

Under Muskie's planned amendment, once a senator was elected he would have to put all his investments in a blind trust from which he could only receive the maximum $8.625 a year. The rest would have to be reinvested.

For Muskie it is a personal thing. Over the years he says he has avoided buying stocks and bonds, or going into land and other speculative ventures offred him on the grounds that there could be a conflict of interest. Instead he has traveled the lecture circuit, earning until recently around $30,000 a year.

Beginning in 1975, Muskie cut back his speaking because an honorarai limit of $15,000 was established for senators. Last year it was raised to $25.000, but under the proposed code it would be cut back to $8.625 if Muskie had no other source of personal income.

"I'm disturbed by the rationale of what the [Nelson] committee has done," Muskie said recently.

"If disclosure of honoraria [which has been in effect for five years] doesn't work to prevent conflicts of interest, why is [disclosure] all that is required to control conflict in investments?"

The Nelson committee report on the ethics code gives Muskie an irinnic answer. In discussing conflicts that could arise with members owning stock in government-regulated companies or firms controlld by committees on which they serve, ht the report said:

"The committee discovered that it is one thing to describe cases which could pose a disturbing fconflict of interest, but quite another to formulate a rule which meets thw worst case without being unreasonable."

Muskie and other Senators, both affected and unaffected by the income limitation rule, plan to argue that the honoraria limit rule is "unreasonable" for them.

"What record is there that reveals conflicts from honoraria?" Muskie asked rhetorically the other day. "There were no hearings on conflicts, no evidence . . . It is foolish, foolish mistake."

Nelson, in an interview, argued that large honoraria gave "the public an image, a serious appearance of conflict. The size of the payments and the committee assignment of those involved."

Muskie countered that a Harris survey, taken for the commission that drafted the House ethics code, showed taht although the public opposed large fees for members' speeches, public opposition sharply diminished when the size and nature of the honoraria were disclosed to the voters in the member's district.

Nelson and his colleagues recognize the problems posed in dealing with income limitations.

The $8,625 earned-income rule, the committee report said, "was the most difficult and probably the most controversial" adopted.

Sensing that additional problems would develop, according to the Senate source, the committee initially distributed only one copy of its resolution and report to each senator on Friday.

No copies were made available to the press, and reportedly only 125 were printed.

Before the report was released, the Nelson committee agreed to postpone final approval of two controversial new rules in its resolution after conferring last week with the Senate leadership and key members of the new Senate Ethics Committee.

A new rule to limit political activity by a member's personal staff was put down for further study. Another tule, creating a means for handling an equal employment opportunities code for the Senate, was referred to the Government Affairs Committee for study.

The remaining provisions in the proposed ethics code are expected to be called up for floor debate Wednesday - barring some new complications. The basic provisions:

A bar on members and staffers earning $25,000 or more from also working for pay in professions such as the law, architecture, engineering and medicine. This provision goes beyond income limitation and may also face amendment on the floor. One proposal being circulated would prohibit a member from practicing his profession in a firm, but would allow him, under the income limitation to act on his own.

Extensive income and gift disclosure provisions for both senators and top staffers. The paperwork, here, is also a possible target for critisim. The 100 members and a n estimated 1,300 staffers would be required to file.

On gifts, for example, each person would have to report the donor of any gift over $35 except those from relatives. Presents from friends would be included.

To complicate the procedure, those reporting would be barred from accepting gifts totaling over $100 in any one year from anyone registered as a lobbyist or associated with a political action campaign fund. That language would, for example, prevent lobbying groups from buying tickets to $100 or more fund-raising dinners and giving them to senators or staffers - a longtime practice.

Former senators would be be barred for one year fron lobbying their former colleagues or Senate sear. Under the code they would have to make public their would have to make public their income for the prior year.

Each senator's disclosure report would have to be audited at least once during his six-year term . However the code would forbid the audit from being done during the year he is up for re-election.

The outside income limitation of 15 per cent would also be applied to staffers earning $35,000 or more.

The exemptions in the Senate from the earned income rule are interpreted much more liberally than a similarly drafted provision in the House.

For example, a senator who received income form a family business would be exempt from limitation unless he made "substantially all of the managerial decisions for the enterprise or [spents] substantially amounts of time with it . . ." Under those latter circumstances, his income would come under the limitation.

On the House side, Rep. David Obey (D-Wis.), chief proponent of the House measure, argued that a member's income would be limited if he performed any personal services for a family firm.