Debate on a new Senate code of conduct, personal and occasionally bitter last week, will heat up again today.

The focus will be on whether invested as well as earned outside income should be limited.

In the draft code, drawn up by a special committee chaired by Sen. Gaylord Nelson (D-Wis.), a limit of 15 per cent of a member's salary ($8,625 at the current $57,500 rate) was applied to outside earned income. Exceptions were made for money received from family business and farms, and from partnerships where the senator's own services were not material to the concern's gross income.

A similar proposal passed the House.

As Nelson puts it, "I feel there is nothing that can be done about the investment income of wealthy people . . . Unearned income is passive income and earned income is not."

The unearned income that would remain unlimited would include not only dividends and interest from stocks and bonds, but also gains and losses from trading stocks, real estate or oil and gas leases or any other business investment.

Although senators have not been required to disclose publicly such holdings or such income some do.A Congressional Quarterly survey of 1973 voluntary income reports showed:

Sen. Abraham Ribicoff (D-Conn.) had gross incomes of over $125,000 in 1972 and 1973 because of capital gains on sales of real estate, and held shares in 13 Washington area real estate ventures.

Sen. Lawton Chiles (D-Fla.) reported extensive real estate holdings and a 1973 income of $112,534, including $66,346 from partnerships.

Sen. Walter Huddleston (D-Ky.) reported 1973 outside income of $38,849 from partnerships in real estate, a shopping center and a radio station.

Sen. Jacob Javits (R-N.Y.) reported outside income of $74,603, including money from oil and gas leases and land development partnerships.

Sen. Alan Cranston (D-Calif.) showed holdings of $419,624 in various properties, with 1973 income including $4,900 in rents and royalties, $1,290 in partnership income, $11,069 in other business income and a real estate loss of $7,188.

Sen. Mike Gravel (D-Alaska) reported that his three-year gross income for 1971-'73, averaged $65,000 and that he and his wife had a $124,500 investment in Mike Gravel Real Estate, Inc.

Leading the fight against any limitations on income is Sen. Edmund S. Muskie (D-Maine), who argues that public disclosures - which has not been required in the past - is all that is needed now. Muskie argued Friday that earned income, and particularly honoraris, were picked out for limitation because they "are the only form of income we have had to report that having reported then we become the visible target."

"My big point," Nelson said last week "is just the general belief on the part of the public, in my judgement, that somehow or other they ought to be some kind of limit on outside earned income."

Muskie retorted, "Are we going to respond to perceptions or are we going to get down to realities?"

Hoping to defeat the income limit, Muskie has proposed applying the 15 per cent ceiling across the board to both earned and unearned income.