The moment of truth for Philippine President Ferdinand Marcos came in a desperate telephone conversation with Sen. Paul Laxalt (R-Nev.) Monday at the White House. "Senator," a weary Marcos asked, "what do you think? Should I step down?"

Laxalt, who felt a rush of sympathy for Marcos, replied, "I think you should cut, and cut cleanly. I think the time has come."

There was a long pause that seemed to last for minutes. Finally, Marcos said softly, in a dispirited voice, "I am so very, very disappointed."

In the end, a defeated Marcos got the bad news that his 20-year rule could not continue from the senator who had brought him warnings from President Reagan four months earlier and who, he knew, spoke with presidential authority.

Laxalt's message to Marcos was the climax of an extraordinary chapter of diplomacy in Reagan's presidency, one that began with the United States trying to steer Marcos toward military and economic reforms, then push him to hold a free and fair election, and finally to ease him out of office.

In the final weeks, it was an episode in which the administration and congressional leaders managed to forestall violence and bloodshed, but often found events reeling unexpectedly beyond their control. And it was a period in which Reagan himself provided a major source of uncertainty about the administration's policy. Yesterday, Secretary of State George P. Shultz said the demise of the Marcos administration and the ascent of Corazon Aquino as Philippine president "is something that the people of the Philippines have done." But months before the Feb. 7 election that plunged the U.S. ally into its current crisis, the U.S. Navy, the Central Intelligence Agency and members of Congress had urged the administration to press Marcos for economic, political and military reforms.

The pressure on Marcos dated to the Aug. 21, 1983, assassination of opposition leader Benigno Aquino Jr., which triggered widespread unrest and forced Reagan to cancel a planned visit to the Philippines. The killing, blamed on Marcos and his loyalists by the opposition, also was a turning point for U.S. policy.

It brought congressional leaders and many administration officials to the conclusion that Marcos would eventually have to give way to a new government. A November 1984 National Security Council report called for influencing Marcos "to set the stage for a peaceful and eventual transition to a successor government, whenever that takes place."

"We recognized that reforms were necessary," presidential spokesman Larry Speakes said yesterday in explaining administration strategy. "First of all, political reforms . . . in the government, economic reforms . . . and military reforms, knowing that the military had been politicized and that it was necessary to increase the professional capacity of the Filipino armed forces . . . to counter the communist insurgency."

But these goals seemed to be in jeopardy when Reagan dispatched Laxalt to Manila last October with what one official described at the time as "the bluntest presidential message ever sent to a friend." Laxalt warned Marcos that the United States was dissatisfied with the pace of military reforms and concerned that the Philippine army was losing ground to the insurgents. The senator cautioned that Americans were disturbed over the government's failure to bring Aquino's killers to justice.

According to informed accounts, Laxalt and Marcos struck a rapport, despite the senator's warnings, and Laxalt emerged from the meetings believing -- as he told Reagan later -- that Marcos would make the necessary reforms. However, some officials said they feared Laxalt was so convinced Marcos would move that he did not adequately warn Reagan on his return about the deteriorating situation in Manila. That failure, they believe, perhaps caused Reagan to cling to Marcos longer than he should have.Reagan told associates this week that he thought he knew Marcos "the man" because of his own meetings with him dating back to 1969, when, as California governor, he visited the Philippines and meetings later when he was president. One senior official said Reagan "felt in his bones" that Marcos wouldn't last, but liked him and never said this directly. Reagan's feelings about Marcos, other officials said, prompted the one major difficulty in U.S. policy in recent months, when the president was at first reluctant to condemn the election fraud largely committed by Marcos forces.

In an interview with The Washington Post Feb. 10, and a nationally televised news conference the next day, Reagan said there was fraud "on both sides" of the Philippine election. His public statements reflected a private reluctance to criticize a man whom he considered a national leader and an important Asian ally in the global contest with communism.

Even yesterday, as Marcos was being flown to Guam and senior U.S. officials were congratulating themselves on the success of their policy, Reagan was said to be ambivalent about the outcome. A senior official said the president, although pleased about the peaceful transition, was "tinged with sadness at what had happend to Marcos."

Despite Reagan's sympathies for Marcos, the administration had maintained pressure on the Philippine leader, calling for "free and fair elections." U.S. observers, including Senate Foreign Relations Committee Chairman Richard G. Lugar (R-Ind.) and White House counsel Fred F. Fielding, found that the elections were marked by "widespread fraud."

Against this backdrop, Reagan's comments about fraud on "both sides" created consternation and confusion for U.S. policy makers. A day after Reagan made this statement, U.S. intelligence reports concluded that Aquino would have won the election if the count had been fair. The next day, a new statement was prepared at the direction of Shultz and national security affairs adviser John M. Poindexter saying that the election was marred by fraud.

But the statement was held up until Feb. 15, after the Philippine National Assembly had formally certified Marcos as the winner. This statement was issued as Reagan was preparing to leave his California ranch, and from then on, a senior White House official said yesterday, "the screws were really tightened."

"Fortunately, at the 23rd hour and the 59th minute, the president reversed field," Rep. Stephen J. Solarz (D-N.Y.) said.

In Manila, Allen Weinstein, head of the Washington-based Center for Democracy and a member of the U.S. elections observer team, showed a copy of Reagan's statement to Aquino. "She was really appreciative," he recalled. "She felt it was a toughening and a tightening, and it was."

After his news conference, Reagan never again appeared on camera to talk about the Philippines. Instead, his advisers steadily escalated the pressure through a series of written statements culminating in the one issued at 5 a.m. Monday calling on Marcos to leave office.

At 1:45 p.m. Monday, Laxalt was attending a briefing by Shultz on the Philippines in S-407, the "secure room" in the Capitol where sensitive foreign policy and intelligence issues are discussed, when a secretary handed Laxalt a note telling him that Marcos was on the phone.

During the next 20 minutes, Laxalt said, Marcos was "reaching for a life preserver." He began by asking whether the message he had received from the State Department saying that the Reagan administration wanted him to step down represented the president's views. Laxalt assured him that it did.

Then Marcos talked about other ways to stay in power -- or possibly share it with Corazon Aquino. He wanted to know if the "transition" to the new government could last until 1987, when his current six-year term would end. He said that the new government would need his services in securing loans from the International Monetary Fund or fighting the communist insurgency.

"He was a desperate man, grasping at straws," Laxalt said.

Laxalt, who said he became "reasonably good friends" with Marcos in October and felt sorry for him, spoke carefully but told Marcos he regarded power-sharing with Aquino as "impractical." Marcos said he wanted to remain in the Philippines. "This is my home, and I want to stay here and die here," he said.

But Marcos also discussed what would happen if he came to the United States, where he feared "congressional harassment." Laxalt said he didn't think this would happen.

Marcos told Laxalt he wanted assurances that there would be no revenge against his family or associates if a new government took over. Laxalt assured him this was negotiable.

The call ended with Marcos asking Laxalt if he would talk to Reagan. Laxalt asked if it would be satisfactory if he discussed the situation with Shultz. "Of course," Marcos replied.

Laxalt told Reagan and Shultz about the phone call during an Oval Office meeting that included Poindexter and White House chief of staff Donald T. Regan. He quoted Reagan as saying "that there would be no dignity for Marcos in power-sharing." While no one in the room knew what Marcos would do, it seemed likely from Laxalt's report that the Philippine president was moving toward the inevitable realization that he must step down.

Laxalt left the Oval Office, slipped into Poindexter's office and placed another call to Marcos. This time he heard a different voice on the telephone. "All the spunk had gone out of him," Laxalt said. "The options for him had come down to staying there in the palace and dying, or taking a walk."

Marcos told Laxalt that he had not slept all night and that he and others in the palace feared that people would break in and kill them all. Laxalt assured Marcos that he had discussed all the matters that had been raised with the president and Shultz. Marcos then asked whether Reagan wanted him to step down "and I indicated the president wasn't in the position to make that kind of representation and certainly not that kind of demand but that he hoped there would be a peaceful transition."

That was when Marcos asked Laxalt what the senator called "the gut question." Laxalt said that his reply "hit Marcos like a ton of bricks . . . . The pause was so long I thought he left the line."