President Carter's chief domestic adviser, in a tough rebuff to party liberals, warned yesterday that new economic "realities" are forcing a pruning of the 1960s social programs, and declared that Carter will reject any demands that he continue expanding them.

In a speech designed to reinforce Carter's new tight budget policy, White House adviser Stuart E. Eizenstat defended the president's cuts as fair and realistic, and cautioned that Americans shoule "not demand what is unattainable in the face of these new realities."

However, in a departure from his prepared text that was clearly aimed at disgruntled parly liberals, Eizenstat told a less-than-pleased audience at the Women's National Democratic Club that "if they [do] demand it, the new realities will have to take precedence."

His remarks were regarded as the strongest indication yet that the White House intends to stand firm on its fiscal 1980 budget cuts. Eizenstat, the most liberal of Carter's key lieutenants, had been the most vigorous White House opponent of any such cuts before.

Carter has approved a $533 billion budget for fiscal 1980 designed to trim the deficit to below $30 billion, as an anti-inflation measure, with sizable increases in defense spending but a sharp clampdown on outlays for most social programs.

The cutbacks were a major issue at the Democratic Party's midterm convention in Memphis last month, but Carter prevailed in a series of votes by about a 60-to-40 edge. Since then, some liberals have protested the cuts, but the White House has made only token efforts to mollify them.

The major thrust of Eizenstat's speech yesterday was that while Carter was sympathetic to the goals of the 1960s programs, he was facing new economic and political constraints and would not be able to continue expanding them.

The domestic adviser asserted that Carter's "dommitments to sccial justice are as strong -- and his accomplishments as meaningful -- as any past Democratic president."

However, Eizenstat cautioned, "every president must face the reality he inherits... If fthe 1960s seem a time of unlimited expansion, the 1970s seem to be a time of more needed restraint and consolidation -- a time to make the programs of the 1960s work.

"The Carter administration is facing new realities nver imagined even 10 years ago," he said, adding that Carter "cannot re-create the 1960s when he must govern with the far-different problems of the 1970s."

Eizenstat's tough stance drew little more than token applause from party regulars in the audience, and during a question-and-answer period after his speech one listener excoriated the administration for planning modest cutbacks in some future Social Security benefits.

But Eizenstat held firm, insisting the cutbacks were not "an assault on the [Social Security] system," as the questioner had charged, but only "necessary reforms" that were similar to the sort of cutbacks Carter would be proposing in other areas.

He also declined to take an opportunity to criticize the Federal Reserve Board's high-interest-rate policies, insisting the administration "doesn't need to take a position" because the Fed is independent. Several times last year, Eizenstat had decried the Fed's interest-rate rises.

On another topic, Eizenstat also denied the White House has plans to abolish the Pentagon's Renegotiation Board, Which reviews military contracts. He said the agency is needed because "we believe strongly that defence contractors are ripping off the American public from time to time."

Eizenstat's tough talk was the latest in a series of herd-line speeches by Carter and other top officials in preparation for the Jan. 22 budget presentation. Several key political advisers have urged the administration to take the offensive on the issue in an attempt to squelch any coming attacks.

Apart from the current inflation threat, Eizenstat listed several other constraints he said were imposed by the new "realities" -- from erosion of presidential power in the wake of the Watergate scandal to the emergence of single-issue, special-interest groups.

"No better example exists," he said, "than the hospital lobby and the money they spent to beat a hospital cost-containment bill favored by large majorities of the American people." He also cited as a constraint the new independence shown by Congress.

Eizenstat said the best thing for Carter to do under such economic and political constraints is to follow "in the path of his Democratic predecessors by being innovative in the situation he finds himself, in adjusting to the realities of the present decade rather than acting as if we lived in the past one."

He said Carter's new budget would do that -- "not by doing all we would like to do if we lived in a bygone era... but by doing as much as we could afford to do now while facing the unhappier realities of today."