White House and congressional budget negotiators last night drove close to an agreement on the major elements of a plan to cut the federal deficit by $30 billion this year and as much as $45 billion next year.

But an agreement will not come in time to avert the beginning of $23 billion in across-the-board budget cuts. They will start today, as soon as the president signs an order required by the revised budget balancing law.

Even if an agreement is reached, congressional leaders will have to overcome significant opposition to the general shape of the $30 billion plan to pass the enabling legislation.

The deficit-reducing plan, which has been the object of nearly four weeks of tortuous, see-saw negotiations, could become final today if the president, speaker of the House and majority leader of the Senate agree on language limiting what kinds of specific taxes can be included in the package. Negotiators also must still settle some differences over how the spending cuts called for can be guaranteed in fiscal 1989, which begins next October.

The budget law gives congress 10 legislative days beyond today to supersede the automatic cuts with a negotiated package, provided the compromise plan reduces the deficit this year by at least $23 billion.

White House chief of staff Howard H. Baker, Jr. said yesterday that the president would not agree to any postponement of the automatic cuts, also permitted in the law. He made the statement after it was clear there was not enough congressional support for a delay in any case.

The initial impact of the cuts will be relatively minor, however, since they do not become fully effective until Congress adopts its full year-appropriations legislation. Many programs for the poor -- such as food stamps -- as well as Social Security are exempted from the spending cuts.

But starting today, a number of budgets will be slashed. For example, payments to health-care providers for the Medicare program will be reduced by 2 percent on an annual basis.

Agricultural subsidies would ordinarily also be affected beginning today, but the Agriculture Department has postponed signups for 1988 crop year programs until the deficit talks are resolved. Dairy farmers, though, will have their payments reduced by 8.5 percent because there is no sign-up period.

In general, domestic programs subject to the cuts will be reduced 8.5 percent, while military spending (except for exempted military pay) will be cut 10.5 percent under the mandatory program, that negotiators hope an agreement will supersede shortly.

Emerging from last night's negotiating session, House Republican leader Robert H. Michel (R-Ill.) said the overall numbers on spending cuts and tax increases for the two years the plan would be in effect "are pretty well set."

Another negotiator said an agreement is "imminent."

However, some details remain to be worked out. House Speaker Jim Wright (D-Tex.) and Senate Majority Leader Robert C. Byrd (D-W.Va.) must still agree to language that prevents the $9 billion in higher taxes from being raised through changes in income tax rates, postponement of rate cuts mandated in last year's tax legislation, sales taxes or linking tax rates to inflation. And Senate negotiators are still demanding that the House agree to provisions guaranteeing that the second-year cuts in spending will be enforced.

The package now nearing completion would cut $30 billion from the deficit in the current fiscal year through $9 billion of tax increases, cuts in military spending of about $5 billion, reductions in discretionary domestic spending of about $2.6 billion, and cuts in entitlement programs of $4 billion. Additional deficit reduction would come from asset sales, user fees, more vigorous tax collections and lower interest costs.

The biggest hurdle may come, however, when the plan is translated into legislation that must pass both chambers. Republicans on both sides were in full revolt over the plan's outlines yesterday, with many arguing that the automatic cuts are preferable.

But other lawmakers predicted that once opponents fully realize the impact of deep, automatic cuts they would change their minds. "When they start looking at what {the automatic cuts} mean, it's going to shock them back into reality," said Rep. Jim Slattery (D-Kan.)

At the White House, Baker said, "I do what the president says, and he says he won't go along with postponing the sequester," using the technical term for the automatic spending reductions required by the law.

Under the budget law, the president must sign an order by tonight implementing the $23 billion in spending cuts if he and Congress have not agreed on alternate means of reducing the deficit.

Last night, negotiators revised the second-year deficit reduction program, deemphasizing tax increases and substituting higher spending reductions.

But even as White House officials and congressional leaders worked on the final details, their problems in selling the agreement to the full House and Senate mounted.

House and Senate Republicans emerged from separate caucuses yesterday reporting massive GOP opposition to the plan, despite Reagan administration entreaties to support it.

Many Republicans said they would prefer the $23 billion in automatic spending cuts required by Gramm-Rudman-Hollings or an alternate proposed by some senators of both parties that would freeze spending at last year's levels.

Sen. Bob Packwood (R-Ore.), one of the negotiators, dismissed the still-evolving package as a "joke" and predicted that it would be "overwhelmingly" rejected by Senate Republicans. "We think some of the savings are phony. And we are paying too high a price in increased taxes for too few {spending} savings," Packwood said.

Many House Republicans agreed, despite a private appeal from White House chief of staff Baker and Treasury Secretary James A. Baker III at a lunch-time GOP caucus.

"I have to conclude that House Republicans are almost unanimously opposed to the agreement in its present form and most would prefer the sequester to this package," said Rep. John Edward Porter (R-Ill.).

The opposition -- one Democratic aide called it "a full Republican revolt" -- mounted despite arguments from the two Bakers that the package is preferable to the automatic cuts because it would allow higher defense spending and send a stronger signal to the world's troubled financial markets that the U.S. government is capable of getting its fiscal house in order.

Asked whether the appeal from the senior administration officials made a difference, Rep. F. James Sensenbrenner Jr. (R-Wis.) said, "we aren't like Rockettes when we perform . . . . We're independent thinkers."

The Republican opposition dismayed some Democratic negotiators, who called on Reagan to put the weight of his office behind it.

"Why should Democrats carry Reagan's water if he's not willing to fight for it," said Rep. William H. Gray III (D-Pa.), chairman of the House Budget Committee.