Led by its Republican majority, the Senate voted unianmously yesterday to spurn two critical aspects of President Reagan's new plan to cut out social Security benefits.

First rejecting by only a one-vote margin an even harsher repudiation proposed by the Democrats, the Sentate went on reord 96 to 0 against any proposal that would "precipitously and unfairly penalize early retirees" or reduce benefits more than "necessary to achieve a financially sound system and the well-being of all retired Americans."

Almost at the same time, House Democats unanimously adopted a statement characterizing Reagan's proposals as an "unconscionable breach of faith, especially with early retirees.

These reversals for the president came even as a the House was confirming his earlier budget victories by finally adopting, 244 to 155, the much-cut budget resolution he supported for next fiscal year.The Senate was expected to follow suit on the budget today.

Reagan last week propsoed, on top of this first round of budget cuts, major new retrenchments in Social Security, the sharpest of which would affect those seeking in the future to retire before age 65. His recommendations would cut future benefits nearly twice as much as required to keep the Social Security system solvent, assuming the economy behaves as anticipated. Such cuts would serve not only to keep the system afloat without new increases in taxes, they would help reduce futuer budget deficits as well.

Jubliant Democrats said the Senate vote amounted to a repudiation of the scope of the proposed cuts, as well as the retirement provision. Almost mischievously, they suggested that the Republicans merely copied the essence of their proposal, which had been tabled by a vote of 49 to 48, with Republicans Lowell P. Weicker Jr. (Conn.) and William V. Roth Jr. (Del.) joining all Democrats in support of it.

The Republican proposal on Social Security, offered by Senate Finance Committee Chairman Rober J. Dole (R-Kan.), was "a rejection of those propoals, tacitly and inferentially," said Minority Leader Robert C. Byrd (D-W.Va.). "We won the point. . . ," claimed Sen. Daniel P. Moynihan (D-N.Y.).

The vote by the Republican-controlled Senate, although only hortatory in nature, amounted to the most important rebuff Reagan has received so far from Congress -- although the White House itself has been backpleading a little in recent days on the Social Security recommendations.

The Senate action came as the Democratic-dominated House gave final approval to a compromise version of Reagan's slashed-back 1982 spending targets that mandate more than $35 million in actual program cuts.

But even as Reagan could by this vote claim victory in Phase One of his budget-cutting drive, House Speaker Thomas P. O'Neill Jr. (D-Mass.) plotted a strategy to salvage some of the threatened programs.

"If it breaks the budget, then that is the will of Congress," said O'Neill as he outlined a procedure under which the House can entertain second thoughts about actually approving the program cuts that it is instructing its legislative committees to draft.

Asked if he was proposing to unravel the whole austerity program, O'Neill smiled and said he was just "giving members of Congress a chance to vote for the programs that truly made America great."

The House vote was 244 to 155, with all but eight of the opposition votes coming from Democrats, to approve a House-Senate budge compromise totaling $695.5 billion, with a projected deficit of $37.7 billion.

The preliminary budget proposes to give Reagan virtually all the spending cuts he wants, along with room for the more than $50 billion, three-year, across-the-board tax cut that he has proposed.

But yesterday's action only completes the first, easiest phase of the protracted, complex budget process. The budget neither makes actual spending cuts nor binds Congress to the tax cut, which is considered unlikely to pass without modification. Actual tax and spending cuts will have to come later, which is what gives O'Neill an opportunity to try to restore some of the money that the first budget resolution proposes to cut.

The budget resolution approved yesterday also raises the spending ceiling to $661.4 billion for the 1981 fiscal year ending Sept. 30, enabling Congress to pass supplementary appropriations to carry the government through the rest of the fiscal year.

It was in connection with that money bill, which the House passed earlier this month and sent to the Senate, that the Senate adopted the statement of opposition to some of Reagan's proposed Social Security cuts.

Also as a part of the 1981 money bill, the Senate is expected to face today yet another of its periodic flareups over curbing federal funding of abortions.

The Senate Appropriations Committee voted to strip all House-approved anti-abortion "riders" from the bill on the theory that the Senate, now in Republican hands, will no longer block changes in laws dealing with abortion. The Senate Republican leadership, anxious to keep emotional social issues from tripping up Reagan's economic program, is backing the committee. But Sen. Jesse Helms (R-N.C.) has served notice he will try to restore the anti-abortion language.

Before the budget vote in the House, Budget Committee Chairman James R. Jones (D-Okla.), who failed in an earlier attempt to modify the document, delivered a blistering attack on it as a "politically convenient mirage for the time being."

But Jones voted for the compromise and pledged to enforce its spending cuts, which puts him at odds with O'Neill.

O'Neill would enforce the cuts in only a technical sense. He would have the committees meet their targets for program cuts but also encourage them to propose amendments to restore money for programs that they consider essential. Thus House members would be voting on specific program cuts rather than on generalities of the Reagan budget, which has proved to be prohibitively popular in Congress.

"We're not going to roll over and play dead and let them [the Republicans] stew in their own juice," said O'Neill, while insisting he was not being obstructionist.

But it was not clear that O'Neill will succeed in his strategy. "In the end, I am not sure you can restore any of them [the cuts]," said Rep. Leon E. Panetta (D-Calif.), a Budget Committee member. O'Neill did not say how much the Democrats might want to restore, but Panetta said it could be about $4 billion to $5 billion in subsidized housing, farm, alternative fuels, health, transportation, education and jobs programs.

Some committees, like Ways and Means, have met or nearly met their targets, although reluctantly. Others like Education and Labor, have indicated they cannot do so.