The Reagan administration abruptly shifted tactics in its approach to Congress yesterday, threatening vetoes if the president's newest budget cuts are not adopted and tossing aside part of last winter's presidential promise to preserve a social "safety net" for the poor.

Office of Management and Budget Director David A. Stockman laid out the new hard line at a news conference, saying "the enforcement tool" to win the new Reagan cuts would be "33 percent plus one," a reference to the number of votes needed in either House or Senate to sustain a presidential veto.

Stockman also said the administration's much-touted safety net was "a term of art" and a matter of "definition." Asked about President Reagan's statement to Congress last February that the programs he had put in the "safety net" category were "exempt from cuts," Stockman said: "I'll check that language out. I don't think it read quite that way." The official weekly compilation of presidential documents confirms that it did.

On Capitol Hill, members on all sides agreed that Reagan will have a far harder time winning passage of the new round of cuts than he did the first round last spring. Republicans were plainly uneasy that the president had produced a new plan for cuts that seemed politically so difficult to achieve.

Sen. Jake Garn (R-Utah), chairman of the Banking Committee, said in a characteristic remark that it was "unrealistic politically" for the president to think that all the government's spending programs apart from entitlements could be cut an additional 12 percent. This is what Reagan asked for in his speech to the nation Thursday night.

"I'll do all I can to support the president but it will be a much more difficult battle this time around," said the conservative Garn, a little tiredly.

Predictions of victory for the president were rare. One came from Rep. Robert H. Michel (R-Ill.), the minority leader in the House, but he was vague on the question of whether the precise program Reagan outlined Thursday could win approval. "We must now complete what we set out to do," Michel said.

Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) said he thought the new cuts would be passed, but then told reporters: "It's really going to be tough, especially for the additional cuts for 1982. It's always tough to go back and take the second cut, the second bite." (Fiscal 1982 begins next Thursday.)

The Republican whip in the Senate, Sen. Ted Stevens (R-Alaska), said that the new cuts could not be approved in the form Reagan proposed.

Sen. Robert J. Dole (R-Kan.), chairman of the Finance Committee, said Congress would want to cut projected defense spending well beyond the $2 billion Reagan has proposed for fiscal 1982. That $2 billion cut would still leave defense spending rising sharply. Michel and others also said they expected Congress to seek deeper cuts in defense.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said he thought the Reagan program was now in "deep trouble." O'Neill said the president would have "tremendous problems" holding the support of the conservative Democrats and moderate Republicans who gave him his big budget and tax victories earlier in the year. Statements from two members of the moderate Republican group known as the "Gypsy Moths" indicated O'Neill was right.

The speaker said he thought Reagan could get out of his budgetary dilemma only by changing his tax-cut policy to raise more federal revenue, thus creating realistic prospects for balancing the budget. But the Democrats won't try to roll back the tax cut Congress enacted last summer, O'Neill said, perhaps drolly, because that would amount to obstructing the president's program.

The predictions of political difficulties seemed to toughen the rhetoric of administration officials. Asked at a briefing if he thought his former House colleagues would swallow the major new cuts in popular spending programs the administration now wants, Stockman expressed total confidence.

"I think it has a very strong practical possibility" of being enacted, Stockman said of the new program of cuts, "because now we are dealing in the appropriations area, where the enforcement tool is . . . 33 percent plus one." He was referring to the number of votes needed in either house of Congress to sustain a presidential veto.

"The possibility of vetoes," said Stockman, would give the White House "far greater leverage to get the results" it sought than it had during last summer's battle over budget reconciliation, "where it took a positive act of both houses by an absolute majority" to get final action.

However, it will still take a majority in each house to appropriate money for government operations starting Oct. 1. If Reagan were to veto appropriations bills and Congress refused to go along with the lower spending levels he seeks, there could be an impasse.

Some Democrats, not surprisingly, expressed open pleasure at the president's new predicament. House Majority Leader James C. Wright Jr. (D-Tex.) said that the Reagan administration's values, as represented by the new cuts, are "deranged."

Senate Minority Leader Robert C. Byrd (D-W.Va.) said Congress had already cut the budget "way beyond what was recognized as waste, fraud and abuse," and that further reductions would cut "to the bone of human programs, decent programs that Americans want."

The key actors in the budget drama made restrained comments yesterday. Rep. Bill Green (R-N.Y.), co-chairman of the Gypsy Moth coalition of moderate Republicans, said he was not prepared to look at more cuts in social programs unless there is "a reasonable rollback in the increased military budget." The Gypsy Moths have already proposed a $9 billion reduction in defense spending plans for 1982.

Green suggested that deals made last spring would be violated by the new cuts. At that time, the Gypsy Moths went along with the White House's budget ceilings in return for concessions on some of their favorite programs. "I would view further cuts in the programs we negotiated on to be a failure to live up to an agreement," Green said.

The first sign of the political maneuvering that will accompany this new round in the budget-cutting crusade was a speedy retraction last night of a mid-morning statement by Treasury Secretary Donald T. Regan that all general revenue sharing would be phased out by 1984. No, the White House explained after dark, the president would not phase out revenue sharing until he had some adequate substitute for it, like a sharing of excise tax revenues with localities.

More political battles will come when the administration specifies the still unnamed cuts it wants to make in big entitlement programs, many of which (Medicare most obviously) appeared on the original list of untouchable "safety net" programs, but now are targeted for reduction.

Stockman revealed yesterday that he foresaw only $2.6 billion in savings from these cuts during 1982 because he reckoned that Congress couldn't enact such cuts until the third or fourth quarter of the new fiscal year; i.e., until at least next spring. That means that he is looking for cuts in the entitlements that would be significantly higher than $2.6 billion on an annual basis. And that means plenty of political controversy.

The most immediate problem for the administration is to win Senate approval of a bill to raise the federal debt ceiling over the $1 trillion level. Regan and White House chief of staff James A. Baker III pleaded with Republican senators yesterday not to add any amendments to this bill, but at least one, William Armstrong of Colorado, said he would anyway.

The Senate did act yesterday on a continuing resolution that will keep the government in business after Oct. 1 and until new appropriations bills for the new fiscal year are passed. It includes a big new tax break for members of Congress.