The third decennial White House Conference on Aging opened yesterday with the lines forming for a potentially bitter fight over the scope and size of future benefits paid to older Americans under the Social Security system.

Health and Human Services Secretary Richard S. Schweiker took a noncommittal position concerning any changes in Social Security benefits in his opening remarks to the more than 2,000 delegates and 1,500 observers who have come here for the conference. But the chairmen of the House and Senate aging committees, Rep. Claude Pepper (D-Fla.) and Sen. John Heinz (R-Pa.), both endorsed maintaining the basic benefit structure.

Pepper, who received a standing ovation when he was introduced and when he concluded his speech--in contrast to the indifferent reception the conferees gave to administration officials--said he will "oppose to the utmost of my abilities any effort to cut benefits from Social Security."

Schweiker's speech to the conference was preceded and followed by an attempt by members of the New York state delegation to alter the rules Schweiker had issued for the conference. The proposed rules change would have permitted the delegates to vote separately at the final conference plenary session Thursday on each of the 14 committee reports that will be made, rather than lumping them together for a single vote.

But conference chairman Constance Armitage refused to recognize Matthew Schoenwald of New York, who attempted to raise the question from the floor, and the proposal appeared to have been put aside by its advocates for the time being.

Schweiker avoided taking any clear position on Social Security. After referring to the "growing difficulties" and the "increasing burden" on the system, Schweiker noted that President Reagan has proposed creating a bipartisan panel to examine the problems and to consider remedies. "It's our hope that a bipartisan spirit of cooperation will prevail in this critical issue, and that together we'll find the best way of preserving Social Security," Schweiker said.

When the issue of Social Security came up before the conference Committee on Economic Well-Being, however, it was clear that discussion of the issue may become highly emotional. The approximately 150 delegate members of the committee heard a strong attack on the administration's economic policies from one of two keynote speakers, Bert Seidman, director of the AFL-CIO's Social Security department.

Seidman referred to the fear that "many, many Americans" had of growing old 50 years ago because of economic insecurity.

"It wasn't the Norman Rockwell painting that some people in this administration seem to think it was," Seidman said, "and the program that put the poor house out of business was Social Security. Today, Social Security is under its most intensive assault in its 46-year history . . . . The voices are more strident and, what is worse, they are voices of those in high places in our government."

Seidman advocated removing Social Security from the unified budget so that it would not be the target of attempts to reduce the federal deficit, establishing an independent board to run the system and using general tax revenues--in addition to the Social Security tax--to finance the system.

Seidman's views were countered by Prof. Michael Boskin of Stanford University, an adviser to the administration on Social Security. Boskin warned the committee that under the current financing and benefit structure, the Social Security system already faces a projected deficit of $1 trillion.

Unless major changes are phased in over an extended period of time, Boskin warned, the country would "careen haphazardly . . . toward an unmitigated social and economic crisis early in the next century."

Boskin said the welfare aspects of Social Security should be separated from its retirement function, and that retirement benefits should be altered to reflect a worker's total payments to the system rather than an average of the worker's highest earning years.