After 2 1/2 months of declaring the allied air campaign against Yugoslavia will work if given time, President Clinton in recent days has begun what advisers describe as a painful confrontation with the possibility that his predictions were wrong -- and that time is no longer on NATO's side.

Recent White House deliberations involving Clinton and senior national security officials, sources said, have focused on a variety of scenarios of what to do if current efforts to broker a deal with Belgrade fall through. Some of the proposals would involve abandoning the strategy of an air-only campaign that the president envisioned to rescue Kosovo and moving to deploy ground troops as well, they said.

At the most ambitious end of the spectrum are discussions of a full-scale invasion of Yugoslavia. More modest proposals include limiting an invasion to Kosovo or, at a minimum, establishing a "humanitarian corridor" to ferry assistance to the hundreds of thousands of ethnic Albanians inside Kosovo and at risk from Yugoslav troops and Serbian police.

All these scenarios, officials said, would involve Clinton within weeks providing clarity to a question that he has steadfastly tried to keep blurred during 72 days of the allied air campaign: What will NATO do if Yugoslav President Slobodan Milosevic does not yield to bombing?

Despite regular reports from the White House about the impact the air war is having, senior intelligence sources said they have seen no conclusive evidence that Milosevic is ready to bow to NATO demands. A decision on what to do if he continues to hold out looms within weeks, military sources said, because of the months-long lead time needed to assemble a capable ground force before the arrival of cold weather, which comes early in the Balkans.

One senior administration official familiar with Clinton's thinking said he is reconciled to the reality that "we need to have an alternative to continued bombing" and "a way of turning stalemate into victory."

But sources said Clinton remains in intensive consultations over what that alternative should be and what is an acceptable cost of pursuing it. All the ground options being discussed would sharply increase political and military risks for the United States and threaten a rupture of the 19-member NATO alliance, whose members have widely varying degrees of support for continued war.

Even within the U.S. military, sources said, there is a deep and increasingly fractious divide over the question.

Gen. Wesley K. Clark, NATO's supreme commander, has recently told congressional leaders visiting him at his military headquarters in Mons, Belgium, and at NATO bases in Italy and Albania, that he believes a ground invasion of Kosovo would go relatively swiftly and that planning for one is the only way to guarantee that NATO goals are reached. He has shared this advice forcefully with the White House, officials said.

But Clark's advocacy is causing rifts between him and Pentagon authorities, some of whom have grown increasingly critical of the NATO commander's handling of the operation and see him as pushing too hard for a ground invasion. The Pentagon's joint chiefs -- whom Clinton is to meet with at the White House today -- harbor deep reservations about a ground invasion, according to a senior officer familiar with their thinking. They regard a land operation as an enormous undertaking, likely to involve up to 150,000 troops, instead of the 50,000 called for if Yugoslavia agrees to a peacekeeping force for Kosovo.

Even with a decision now, assembling a ground force large enough to invade the rebellious Serbian province could take the rest of the year, one senior defense official said, pushing the timing of a ground operation into next spring. White House officials emphatically reject the suggestion that an invasion would take this long.

Other options developed by Clark's aides offer the possibility of a faster buildup and an invasion by the autumn. But some senior Pentagon officers question the feasibility of these plans -- or even whether they are necessary at this point.

"I think there's a pretty strong belief that the air campaign is working, and we can do it by air, and certainly we ought to let the air campaign go longer before moving rapidly to a decision," one senior defense official said, reflecting the joint chiefs' views.

Another senior U.S. official familiar with White House deliberations agreed that policymakers continue to have flexibility -- and potentially more time -- to weigh options. Only the most aggressive ground options require an imminent decision about massing forces.

The diplomatic implications of an invasion loom as large as the military ones. Within the alliance, only the British are enthusiastic about assembling a ground invasion. Because of opposition by Germany and some other nations, administration sources cite the possibility that an invasion would have to occur under U.S.-British auspices.

In addition, outrage over an invasion would surely echo around the world, affecting relations with nuclear powers such as Russia and China.

"The implications are global," said Chester Crocker, a former diplomat now at Georgetown University, who supports the ground option nonetheless. "Breaking up a sovereign nation at gunpoint is an act of coercion . . . that will polarize NATO within camps."

For two months, the White House's hope has been -- and remains -- to avoid the unpalatable implications of this choice. Senior White House officials said they remain confident that Milosevic's forces are hurting far more than is generally understood by skeptics in Congress and elsewhere. They say they consider it likely that he will eventually fold his hand and accept NATO's demands for an autonomous Kosovo, protected by an international force with NATO at its core.

Administration officials said the fact that Milosevic is meeting with a diplomatic team -- Russian envoy Viktor Chernomyrdin and Finnish President Martti Ahtisaari, representing the European Union -- shows he is looking for a way out.

"There is no way he is going to be able to take the punishment he is receiving every single day," said one White House official. "He cannot last."

But sources said senior national security officials are confronting several factors forcing them to reevaluate. One is Clinton's judgment that a diplomatic settlement that falls short of NATO's demands -- what some officials have derisively called the "half-a-loaf scenario" -- would be a political and foreign policy disaster that the president will not accept.

Beyond that, sources said, there is a logical conclusion to a bombing campaign unless alliance policymakers are prepared -- as they plainly are not -- to expand the war beyond targets of military value and instead aim to broadly terrorize the Yugoslav civilian population from the air. Some administration sources said the logical end of the campaign, under its current target limitations, is nearing within weeks.

Finally, there is the most irreversible fact of all -- the changing seasons. Administration officials said within a month or so preparations will have to be made to make refugee camps in Albania and Macedonia suitable for winter. If the decision to do that is made, Clinton hopes to be ready to also demonstrate that the alliance is taking steps to ensure that the current crisis will not drag on indefinitely. Some officials believe that the mission in Belgrade amounts to a last chance for diplomacy.

At a speech to the graduating class of the Air Force Academy in Colorado Springs yesterday, Clinton reiterated NATO's determination to prevail. "Some things are worth fighting for," he declared.

Clinton announced that the United States will deploy an additional 48 combat aircraft to NATO operations in Yugoslavia and said the United States will contribute approximately 7,000 troops to a 50,000-member international peacekeeping force that would police Kosovo if Milosevic agrees to withdraw his troops from the Serbian province and allow the return of displaced refugees.

But administration officials said those troops are "dual use" and could be used for combat purposes if needed.

Officials at the Central Intelligence Agency, sources said, are among those telling Clinton that ground forces may be needed.

"Generally, after a little more than two months of bombing, there's no question that Milosevic has been hurt; the situation is worse for him," a senior U.S. intelligence official said this week. "At the same time, we don't yet see signs that he's about to break or that he's ready to yield to NATO's core demands. The balance is roughly that. He has been hurt, but he is still holding, for the most part, on his core demands."

More optimistically, other intelligence officials said there is evidence that aides around Milosevic are telling him that the country is suffering too much to continue on the present course.

"He has for the most part surrounded himself with yes men, but we've had some indication that people are getting through to him to say, take a look around you, here are some reports from the scenes of destruction," an intelligence official said. "But we have no evidence that he's lost his core belief -- this is on the other hand -- that NATO will falter before he does."

The other factor Clinton must weigh in assessing how much time he has is domestic support. Most polls suggest his approval rating, as well as support for an extended air war, has faltered in recent weeks. Skepticism abounds in Congress about the efficacy of the air campaign, but so does opposition to a ground invasion.

Even so, some members of congressional delegations back from Europe said they believe the time has come to consider ground options.

"If anything, I was swayed that air power was not going to be able to do what they want it to do," said Sen. Tom Harkin (D-Iowa), who visited the region last month. "If they are sticking with the goals they announced . . . it's going to take the use of mechanized ground troops soon."

Rep. David L. Hobson (R-Ohio) came back from the region last week convinced that the ground troop option should never have been taken off the table. But he faults Clinton's lack of leadership in preparing the public for such a costly endeavor.

"My district is not prepared nor is the Congress of the United States prepared to risk American troops in Kosovo," he said. "The president has failed dramatically in that regard."

No one from the White House, Hobson and Harkin point out, has asked the congressional delegations who have visited the region for their views on the matter.

Clark's effort to open the discussion on ground troops also is supported by top Air Force commanders involved in the war. They expressed doubts in recent interviews that air power alone can force Milosevic to the bargaining table, or even that NATO warplanes will be able any time soon to find and demolish the dispersed Yugoslav troops and equipment that remain in Kosovo without unintentionally striking civilians who are often mixed in with them.

"We have no clue how many precious targets Milosevic has, or when he'll fold," said one top NATO airman.

Even without a decision on ground troops, Clark and military planners are trying to tweak the air campaign in Kosovo so that if troops were ordered in without a peacekeeping deal, they would find a landscape largely devoid of well-equipped Yugoslav forces who could mount a coordinated defense.

Staff writers Dana Priest and Vernon Loeb contributed to this report.

CAPTION: The president is discussing several possible options should current talks fail to bring peace.