House Democratic leaders yesterday backed off a bill that would postpone the effective date of the new Ethics in Government Act for the benefit of recently fired administration officials.

The bill was scheduled to come up yesterday, under a procedure that requires a two thirds vote for approval. The bill's manager Rep. George Danielson (D-Calif.), said leaders decided to postpone action because press reports about the bill 'scared some people." He also said, "I told the speaker I smelled around and we would have trouble getting the two-thirds vote."

Danielson said his Judiciary subcommittee would not hold a hearing on the bill and it would be brought back, probably, in September, under regular procedures, which would require only a majority vote.

The Bill, introduced late last week by Danielson, was put on the schedule by Speaker Thomas P. (Tip) O'Neill (D-Mass.) without committee consideration.

O'Neill said he had the bill drafted and scheduled after some top-ranking officials came to see him last Thursday. But he denied vehemently that any fired Cabinet Secretary was involved. Instead, he said these were high-ranking aides to Cabinet members swept out in President Carter's shakeup. O'Neill refused to identify them, however. "That's for me to know and you to guess," he said at his daily news conference.

O'Neill said the aides had been "summarily dismissed. I just think in decency we ought to protect them. They thought they could lose their livelihood.

The ethics law, which went into effect July 1, attempts to slow the "revolving door" of agency officials leaving to work for a private firm and using their inside contacts to lobbly their former agency.

It prevents former officials ranking GSI6 and above from personal contact with their agencies for two years on matters in which they played a substantial and personal role.

The presumption is that the law makes it more difficult for former officials to get a job in fields using their government expertise.

Bureaucrats have complained mightily about the law, some threatening to resign if it were not softened. Accordingly, before it took effect, Congress exempted those in scientific and educational fields and some others and relaxed the prohibition on "aiding and assiting."

The Danielson bill would postpone the effective date from July 1 to Sept. 30. The citizen lobby Common Cause objected strenously that it would not only cover the few fired officials but "remove the cover" from thousands now affected by the law. "What this says is that the laws can always be manipulated to accommodate the powerful," Common Cause said in a letter to congressmen.