The multimillion-dollar lobbying and advertising campaign aimed at keeping federal and postal workers outside the Social Security blanket may be backfiring on Capitol Hill.

Many members of Congress who like feds (there are some) have quietly urged union leaders to stop fighting against Social Security coverage.

They want them to concentrate on "winable" issues, such as preserving civil service retirement rules and getting U.S. workers a modest raise this fall.

The friendly legislators and staffers, who don't want to anger union chiefs, say the nationwide radio and newspaper ad campaign is alienating Congress and the general public.

In private conversations they say the ads are angering and frightening feds unnecessarily by implying that extending Social Security coverage to new federal employes will cause the civil service retirement program to go broke.

A Democratic subcommitte chairman said the ads, focused on this one issue, have "created a climate of hysteria" in the federal work force. That climate, he said, makes it harder for friendly members to get colleagues' support for an October pay raise and to block administration plans to raise the retirement age for government workers.

Members of the American Postal Workers Union ticked off House Speaker Thomas P. O'Neill on Wednesday when they showed up en masse at his Capitol Hill office demanding an appointment. O'Neill, who supports putting new federal workers under Social Security, refused to meet with the big group. He did talk with a five-member committee.

"There were groups of postal workers here all day. Some of them were very nasty and had to be asked to leave," an employe said. APWU leaders managed to head off a an office sit-in planned by some New England union members hoping to embarrass the speaker. O'Neill, as the Democratic leader of the House, is not the best person in the world to anger if one has business before the Congress.

APWU brought 4,500 members here this week to lobby Congress. Other unions have had people in town, or are planning to bring them here to lobby on a number of issues--but primarily to oppose universal Social Security coverage.

The House is scheduled to vote Wednesday on a bipartisan plan to shore up the financial condition of Social Security. One part of the plan would put federal workers hired after next January under the Social Security system, which covers most other American workers.

"Social Security coverage is inevitable. It is as simple as that," said a key congressional aide. "When new federal workers go under Social Security, there will be a supplementary civil service system for them. They would keep paying into the main fund and it will not go broke. It is misleading and misinforming people to say the civil service system will go broke if Social Security happens."

A congressional aide whose boss has always supported feds said "the Social Security thing has become a monster. It has drained energy and effort people should be making in other areas. A group of postal workers came in here Wednesday and it was like a cartoon. They kept jamming in the office, more and more of them, and some of them were very nasty. That is not the way to lobby!"

A union lobbyist said he detected little resentment from members of staffers "and I think they would have told me. You know what I think it is? I think they are afraid we will make them vote for a one-year delay [in putting feds under Social Security] and they want to get it over with this year so they won't have to deal with us in an election year.

"What it is," he said, "is that Congress is just not used to federal workers lobbying and being militant. They don't like it. You don't hear them telling the oil lobbyists and the big corporations that they are pushing too hard, do you?"