With older people leading the way, Americans are expressing sharp opposition to the Reagan administration's proposed changes in the Social Security program, according to the findings of a new Washington Post-ABC News poll.

By a ration of 3 to 2 among those expressing an opinion, citizens disapprove of the plan that Reagan advisers say is aimed at rescuing Social Security from bankruptcy. By almost 2 to 1 among those expressing an opinion, individuals say they feel they would be hurt rather than helped by the proposals.

The sentiment revealed in the poll, the first scientific sampling of national reaction to the proposed changes, shows the public apparently ready to rebuff President Reagan for the first time in his attempt to reorder national economic priorities. If other polls reach similar findings, the result could be an emotionally charged battle between Reagan and the Congress.

Until now, many Democrats on Capitol Hill have said that they opposed much of Reagan's economic recovery program but were voting with the president because he clearly commanded overwhelming public support. With leaders in Congress already expressing opposition to the proposed changes in the Social Security program, public resistance to it could well prod Congress into a confrontation on an issue in which it, not the president, enjoys public backing.

The administration proposal, as outlined by Health and Human Services Secretary Richard S. Schweiker Tuesday, calls for a gradual change in the Social Security benefit formula so that people retiring in the future would get somewhat less in initial benefits than under current law. In addition, there would be sharply reduced benefits for people who choose to retire before the age of 65, and it would be more difficult for people to become eligible for disability insurance.

On the other side, the program outlined by Schweiker would allow people over age 65 to earn an unlimited amount of money without losing any Social Security benefits. Under current law, there is a limit of $5,500 on the amount a person may earn without having benefits reduced.

The public, as shown in the Post-ABC News poll, strongly approves that last change, by 67 percent to 28 percent, with 4 percent undecided. But equally strongly, it disapproves reducing initial benefits, by 66 percent to 28 percent, and it is against reducing benefits for early retirees, by 59 to 38 percent.

A total of 1,003 people were interview nationwide by telephone on Thursday in the new poll. Only 8 percent said they felt the Social Security system to be in "good financial shape," with the rest saying it was either in "bad financial shape" or somewhere in between.

For the vast majority of Americans, the way to maintain the system is neither to decrease benefits nor to raise payroll deductions. Given those two options along with a third choice -- protecting Social Security "at all costs even if money has to be taken from other government programs" -- two out of three choose the last method.

The poll suggests that the Reagan administration move on Social Security could have political ramifications for the president that go beyond the success or failure of his attempt to restructure the program. Overall, Reagan remains highly popular, with 66 percent of those interviewed saying they approve his performance in office. That figure represents a 7 percent drop since the last Post-ABC News sounding about a month ago.

While the decrease may not be considered substantial, almost all of it may be attributed to a sudden decline in the president's popularity among older Americans, those most conscious of any threat to retirement benefits.

Among retired people who receive Social Security benefits, for example, only 54 percent said they approve of Reagan's handling of the presidency. Until now such people have been regarded as the heart of the Reagan constituency, and have rated him at least as high as the rest of the population in public opinion polls.

While the Social Security proposal apparently does not affect the high regard younger Americans have for Reagan, they nevertheless express strong opposition to the proposed changes.

Those aged 18 to 29, for example, were just as opposed to the proposals as were those already on Social Security or those about to enter the program. Among retired people receiving benefits, the changes were opposed by 50 percent and approved by 31 percent, with 19 percent expressing no opinion. The figures for the public at large are 49 percent disapproving, 33 percent approving, and 18 percent expressing no opinion.

Further, despite statements by Reagan advisers that the new proposals do not violate any Reagan pledges, a great many Americans seem to disagree. Asked whether they tend to agree or disagree that "Reagan is breaking a campaign promise to voters by proposing cuts in future Social Security benefits," 39 percent say he is breaking such a pledge, 39 percent say he is not, and 22 percent express no opinion.

Just how important old-age benefits are to most Americans is underscored by the response to another question in the Post-ABC News poll. One bit of the conventional wisdom in 1981 is that the public is demanding a strong defense posture and is backing the president in his request for vast increases in military spending even as he plans some $50 billion in cuts in other government programs.

That belief may be correct for many of the social programs Reagan wants to cut, but it apparently does not apply to Social Security. The Post-ABC News poll asked this question: "Do you agree or disagree: Reagan should not seek cuts in future Social Security benefits at the same time he is asking for substantial increases in military spending." A total of 50 percent said they agreed, 44 percent disagreed and 6 percent expressed no opinion.

How the Survey Was Conducted

A total of 1,003 adults nationwide were interviewed by telephone Thursday in Washington Post-ABC News poll of attitudes toward the administration's proposed changes in the Social Security system.Theoretically, figures based on a sample that size are subject to a sampling error of about 3 percent in either direction, 95 percent of the time. Figures based on smaller groups within the sample are subject to a slightly higher margin or error.

A special attempt to interview people aged 50 and older was made, so that about half those interviewed fall into that age category. Such an "over-sample" permits reliable analysis of a group that might otherwise be too small.