The Senate Rules Committee junked key portions of a proposed committee reorganization plan yesterday, than got into a squabble about how to write a code of ethics for the Senate.

The committee, under a mandate to report back to the Senate by Wednesday on reorganizing its sprawling committee system, voted to keep in existence as separate committees the Senate Veterans Committee, the ethics committee, the Small Business Committee and the Joint Economic and Joint Internal Revenue Taxation committees.

All five had been recommended for extinction and absorption into other units by a special study committee headed by Sen. Adlai E. Stevenson (D-Ill.), which had proposed reducing the Senate Committees from 31 to 15, in order to streamline operations and end overlapping. Yesterday's decisions mean there will be at least 20 committees.

The day wasn't a total loss for the Stevenson plan, however. The Rules Committee agreed to kill nine existing units that Stevenson's group wanted killed.

They included the Senate District of Columbia and Post Office and Civil Service committees, to be absorbed by a new Governmental Affairs Committee (Government Operations); the Space Committee (to Commerce), the Nutrition Committee (to Agriculture), the Committee on Aging (to the new Human Resources Committee); and the Joint Atomic Energy, Congressional Operations and Defense Production committees, whose functions will be distributed to various committees.

From the start there was little doubt that the Veterans Committee would be retained because of lobbying pressure from groups representing millions of the 29 million veterans in the nation. They had lobbied heavily to get the Rules Committee to keep the veterans unit separate to look after their needs, rejecting proposals that jurisdiction over veterans' legislation go to Human Resources of Armed Services. Sen. James B. Allen's (D-Ala.) motion to retain te separate unit was approved by voice vote.

Retention of a Small Business Comittee, as requested by hundreds of thousands of firms with 43 per cent of the nation's business output, was a tighter fight. A move to unite this committee with the powerful Finance Committee failed on a 4-to-4 tie, and then a move to put it in Banking lost 5 to 4, with Allen, Sens. Claiborne Pell (D.R.I.), Harrison A. Williams (D.N.J.), Dick Clark (D-Iowa) and John Tower (R-Tex.) in the majority. The committee then unanimously voted to keep it independent.

There was a similar close fight over wiping out the Committee on Aging, with Williams arguing that it was an invaluable study unit representing the nation's 22 million elderly. He opposed shifting its functions to his own Human Resources Committee, saying the aged were a better served with a separate study committee, but lost, 5 to 4, with Clark, Allen and Pell also on the losing side.

The other changes were made with little discussion, but when Clark proposed adding language to instruct the ethics committee to report back a financial code of conduct within 30 days, Senate Majority Leader Robert C. Byrd (D.W.Va.) demurred, saying such a code could better be handled by a special ad hoc committee to be appointed by the leadership.

Clark argued that the best and fastest way to get the code was to tack it onto the committee reorganization plan, but the senators were still squabbling over the issue when the committee closed shop until today.