President Reagan, struggling to shape his new and most divisive budget cuts, said yesterday he will outline his decisions in a nationally televised speech Thursday at 9 p.m.

The president initially planned to make the speech Wednesday, but needed the extra day to nail down the final parts of a proposal that many members even of his own party think takes too much from social programs and not enough from defense.

Treasury Secretary Donald T. Regan reiterated yesterday that the administration is firmly committed to $16 billion in new cuts of which no more than $2 billion would come from defense.

Reagan and his advisers are counting on the speech to restore the momentum the president had in the early summer when he rallied popular and congressional support for his first round of budget reductions.

The mood has changed, however, and yesterday Republican leaders were running for cover, hoping that the president would soften his plan, particularly the contemplated postponement of cost-of-living increases in Social Security and eight other programs. Democrats, meanwhile, were smiling.

"I think everything is going ahead pretty good. You know last June we were dead," House Speaker Thomas P. O'Neill Jr. (D-Mass.) said. The people like Reagan, he added, "but they're not buying his program." O'Neill and Senate Minority Leader Robert C. Byrd (D-W.Va.) enjoyed throwing back at Republicans a word used frequently last spring to describe their party--"disarray."

As Reagan worked to shape the program of cuts and his speech, there were these other development:

The food stamp program was estimated to be running $1.5 billion over the $10.6 billion cap the administration proposed and Congress voted as part of the budget cuts earlier this year. Administrator William Hoagland said an across-the-board cut in individual benefits may still be needed on top of a request to Congress to slice another $700 million by eliminating the cost-of-living increase due next April.

Sen. Robert T. Stafford (R-Vt.), chairman of the education subcommittee, announced that he will oppose any further cuts in three large education programs--for the disadvantaged and handicapped and for college students. He also said he will oppose early abolition of the Education Department, which Reagan is reported to be considering.

The Republican chairmen of the National League of Cities and the National Governors Association warned Reagan that he would be breaking a campaign pledge and making a serious mistake by calling for a phaseout of general revenue-sharing.

Indianapolis Mayor William H. Hudnut III and Vermont Gov. Richard A. Snelling, both describing themselves as Reagan loyalists, were attempting to head off a plan before the president to cut $320 million from revenue-sharing funds in fiscal 1982 and phase out the program over the next two years. Hudnut recalled Reagan's campaign statement that "reenactment of revenue-sharing will be among my highest priorities" as president.

It was reported that the 12 percent cuts in appropriations the administration is planning would be from the president's March budget requests rather than amounts subsequently voted by Congress, and therefore, in some cases, would be cuts much larger than 12 percent. For example, Reagan in March requested $720 million for the so-called WIC program that supplements the diets of mothers, infants and children, and a 12 percent cut would reduce it to $633 million. Congress, however, this summer approved $1.017 billion for WIC.

Reagan met for about 90 minutes yesterday with Office of Management and Budget Director David A. Stockman and the top three White House advisers, presidential counselor Edwin Meese III, chief of staff James A. Baker III and deputy chief of staff Michael K. Deaver. Reagan asked Stockman to get him some additional details that will be considered in a final set of meetings today.

White House deputy press secretary Larry Speakes said that the president and his aides were making decisions right down to the wire. "The final shape of the program has not been determined," he said after the meeting yesterday morning.

The most sensitive issue was whether Reagan would hang tough on his announced intention to cut cost-of-living increases in Social Security and eight other benefit programs, a decision that could have grave political repercussions.

Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) and House Minority Leader Robert H. Michel (R-Ill.) told Reagan Monday that such a program would not win congressional approval.

One of the few Republican voices still speaking loudly in defense of the cost-of-living cuts was Sen. Pete V. Domenici (R-N.M.) who telephoned Reagan yesterday morning. "I told the president to hang tough," the Senate Budget Committee chairman said later. The problem "won't go away . . . it won't be easier next September or October" in an election year, Domenici said.

Sen. Jake Garn (R-Utah) complained that people are giving up too soon. "Everybody is panicking on the whole thing . . . they aren't waiting for it to work," he said of the president's economic program.

Garn added, however, that there is no magic to the administration's prediction of a $42.5 billion budget deficit in 1982, indicating that a deficit of $60 billion would not be too bad.

Regan, however, told the House Budget Committee that $42.5 billion is still the target. "To restore the confidence of the financial markets and the public we must take action to prove that the deficit is under control. We must continue to specify and adopt the spending reductions needed to bring the deficit down to its target level of $42.5 billion in fiscal year 1982 and into balance in fiscal year 1984," he said.

Regan dismissed suggestions that the administration consider postponing parts of the Reagan tax cut bill or enacting new revenue-raising measures to control the deficit.