Soon after learning that President Reagan would undergo major surgery, Craig L. Fuller, chief of staff for Vice President Bush, telephoned California for the advice of veteran Reagan political consultant Stuart K. Spencer.

Spencer's counsel to Bush was simple: consult every day with First Lady Nancy Reagan and do what she says. Bush took the advice without hesitation.

The call to Spencer underscores the low-key, deferential role that Bush has sought to play in the midst of a crisis that triggered the first transfer of authority to the vice president since the 25th Amendment to the Constitution was ratified in 1967.

Bush performed loyally and out of the limelight -- a posture that has been a source of both praise and criticism throughout the Reagan presidency. While the White House chief of staff, Donald T. Regan, adopted a highly visible and prominent role in the days before and after the surgery, Bush stayed behind, asking Nancy Reagan what she wanted him to do.

This reticence, characteristic of Bush, is a topic of much political speculation as jostling intensifies for the 1988 Republican presidential nomination. Critics say Bush's penchant for a low profile is a liability in a prospective contest with Rep. Jack Kemp (R-N.Y.) and Senate Majority Leader Robert J. Dole (R-Kan.), but Bush is described as unwilling to do anything else.

"George realized it would be a mistake for him to change his ways during a crisis and try to hog the limelight," a Bush associate said. "What he has going for him is that he's a dutiful and fully loyal vice president. He's not going to change."

Bush was also -- at times -- inclined to follow his own instincts. Against Regan's recommendation, the vice president left his vacation estate in Kennebunkport, Maine, on the Saturday of Reagan's surgery and rushed back to the capital for the transfer of power.

Events surrounding that turnaround were one of the major irritations of the last week for Bush, officials said. He was chagrined when the White House announced that he was spending the weekend in Maine, because he was already thinking about returning immediately. And the explanation that was given by the White House the next day for his return -- his "personal relationship" with the president -- was not the only reason, officials said.

On Thursday, the day before Reagan's trip to the hospital, Bush was given a medical briefing by Dr. John Hutton, the deputy White House physician. At that point it was not known that Reagan would require major surgery, but Hutton mentioned that the president's brother had recently had such intestinal surgery, according to one informed administration official.

Bush decided to proceed with a previously scheduled trip to Boston for a fund-raiser, speech and other events. At each stop, his staff checked with Washington. Late in the afternoon, Regan telephoned with the news that a growth had been found in the president's colon and surgery was scheduled for Saturday, the official said.

Regan recommended that Bush continue on to Kennebunkport; Barbara Bush had already left by car for Maine. "There was no reason not to go on," said the official.

At 6 p.m. Friday, White House spokesman Larry Speakes, acting under Regan's instructions, told reporters that Bush was "spending the weekend at Kennebunkport" but would be in touch with the White House.

Even as his motorcade headed out of Boston and on to Maine, Bush was pondering a return to Washington, the official said. As soon as Bush aides heard the White House announcement, "I knew it was a mistake," said one. "He never agreed to stay" for the full weekend.

This aide said it was a misunderstanding by both the White House and Bush staffs.

During the two-hour drive to Kennebunkport, Bush listened to news about Reagan's surgery on the car radio. He decided that the "prudent" thing to do would be to return to Washington. Bush knew at that point that the White House was prepared to transfer Reagan's authority to him when the president went under anesthesia, the official said.

Bush said Reagan is like "a brother" to him, and he wanted to return out of personal concern for the president, the official said. But the vice president also realized that it would look frivolous to be serving even for a few hours as "acting president" on the coast of Maine when he could fly back to Washington in a hour or so.

Friday night in Kennebunkport, there were repeated telephone conversations with the White House. Regan urged Bush to remain, officials said, if only because it had already been announced that he would, and an abrupt return might unduly alarm the American people.

But Bush and his advisers thought that was not a major problem, since attention would soon shift to the president's operation and his condition. They were also mindful of possible "worst case" scenarios -- just in case something went wrong during the operation, officials said.

By the last phone conversation, one official said, Regan was coming around to Bush's view that the vice president should come back the next morning. Regan was "positive" about it Saturday morning when Bush left Maine.

It was announced then that the sole reason for Bush's return was his personal concern for Reagan. "The statement sort of made it appear that Bush hadn't known enough on his own to come back immediately, when his first reaction was to do just that," one official said.

At 10:40 a.m. Saturday, Reagan signed a letter transferring power to Bush, effective when he went under anesthesia. That happened at 11:28 a.m.; Bush was in the air.

Bush was not notified by Regan that he was the "acting president" until 22 minutes later, at 11:50 a.m., when he landed in Washington.

According to one informed official, the explanation for the delay is that the surgery was scheduled to begin at about noon, but unbeknownst to Regan it started a little earlier.

In the days that followed, Regan and the First Lady stayed close to the president. Mrs. Reagan was told by doctors to limit visitors to her husband; Bush complied with her wishes.

On Monday, when Reagan had been scheduled to address a diplomatic reception at the White House, the vice president consulted with Mrs. Reagan and learned that she wanted to stand in for her husband. Bush attended the reception, but it was Mrs. Reagan who gave the principal speech.

Throughout the week, Bush also continued to follow the advice of Reagan advisers and political strategists. Officials said he talked to Treasury Secretary James A. Baker III, the former White House chief of staff and Bush's 1980 campaign manager, and former deputy chief of staff Michael K. Deaver, the longtime intimate of the Reagans who now operates a public relations firm here.

Bush was not invited to the hospital immediately to visit the president, and this led to widespread comparisons with Regan, who was there often in the first few days. Associates of Bush said he was respecting Mrs. Reagan's wishes.

Bush was supposed to visit the president Monday, one official said, but it turned out Reagan was asleep at the time and the vice president decided to stay at the White House.

Bush made his first visit Wednesday, and later in the day the White House issued an official photograph, published in newspapers across the nation. It was a photograph that reflected the weeklong prominence of chief of staff Regan.

Officials said that Reagan was initially photographed with Bush alone. But then Regan arrived for a meeting with the president, and all three were photographed together, laughing. Various photographs were prepared for release -- some with Regan, and some with just the vice president. According to two informed officials, Regan chose to make public a photograph that included himself, not just the president and vice president.

The selective release of such photographs is a routine practice at the White House, but it seemed to carry a heightened symbolic importance this time. Regan has adopted a more assertive style than his predecessor, Baker, who often stood aside in such situations.

Although Mrs. Reagan was said to be relying on Regan closely at the time of the surgery, two officials said she was not pleased with later news reports describing in detail the chief of staff's role.

Many White House officials and Republican political strategists were discussing among themselves last week whether Bush had been pushed out of the limelight by Regan. But Bush, content with a low profile, apparently was not concerned. He went out of his way to praise Regan publicly for the way he was running the White House.

On Friday, Bush ended the week the way he had started it. When it was announced that the president would return to the White House Saturday, Bush called Mrs. Reagan about the welcoming ceremony. Bush said he could stay if Mrs. Reagan wanted him to, but his grandchildren were in Kennebunkport and he wanted to visit them.

Nancy Reagan urged him to go to Maine, and he did.