White House chief of staff Donald T. Regan has both consolidated his power and demonstrated the difficulty of using it following President Reagan's surgery last weekend to remove a cancerous tumor from his colon.

In the four days since the president entered Bethesda Naval Hospital, Regan and Nancy Reagan have worked closely together to control almost all access to the chief executive, pace his recuperation, and take care of such minor items as choosing the photograph in which the nation got its first glimpse of the recovering president.

"It's just him and her managing it," said a White House official. Another said, "He's an older man and he was comforting to her" in a period of emotional strain.

A blunt-spoken former Wall Street tycoon and Treasury secretary, Regan, 66, has been through a string of crises in the six months since he arrived at the West Wing, but is described by aides as undaunted -- and determined to forestall the Reagan second term from slipping into lame-duck status.

However, recent days have found Regan struggling to advance the president's policy goals in Congress and coping with questions about his own prominent role in the president's absence. Some of those questions have been raised by his colleagues in the White House, where Regan's efforts to assert his dominance since the president's surgery are widely seen as the principal subplot in this drama.

As part of his growing control over the White House apparatus, which began long before the president's surgery, Regan has also appointed a former White House lobbyist, Dennis Thomas, to be a leading assistant, filling a role not originally included on Regan's corporate-style organization chart.

Sources say Thomas is expected to take on many of the duties that were handled by former deputy chief of staff Michael K. Deaver, such as scheduling and White House administration, and may also oversee the White House personnel office.

Regan turned to Thomas when he realized that he was being inundated with details, officials said. "Regan's learned there are limits on his time and what he can do himself," said a longtime associate.

Regan has had to deal with significant political trouble. Senate Republicans are still fuming over the White House decision to abandon their position on Social Security cost-of-living adjustments last week, a decision that led Majority Leader Robert J. Dole (R-Kan.) to say that Regan didn't understand how important budget deficits are. Regan was outraged, thinking "it was a low blow, unkind, unfair," said a White House official.

The chief of staff rebuked one assistant, political director Edward J. Rollins, when Rollins reported on the grim Senate mood, an official said. Rollins has said he intends to leave the White House later this year.

The role of another Regan assistant, communications director Patrick J. Buchanan, has also been diminished, at least compared to expectations raised when he joined the White House about his powerful influence there. "He's just another presidential assistant," said one official.

A third Regan lieutenant, legislative liaison Max L. Friedersdorf, has likewise assumed a lesser role at a time when Reagan's relations with Congress have growing increasingly difficult, according to several White House officials.

Among the White House staff, officials said, only national security affairs adviser Robert C. McFarlane carries as much weight as Regan, and there has been friction between them at times. Regan was the only adviser at the president's side in the days after the surgery.

Vice President Bush visited for the first time with Reagan yesterday, and the White House issued an official photograph of the occasion. McFarlane is expected to make his first visit today.

Just hours after the surgery, it was Regan who informally "tested" the president to determine if he was ready to reclaim the presidential authority he had transferred to Bush earlier that day. It was Regan who was with the First Lady when news of the malignant tumor was brought to her.

Today, it will be Regan taking recommendations for a new director of the Office of Management and Budget to the president's hospital suite. Regan has said he wants a director with less independence and visibility than David A. Stockman, who resigned, and a leading candidate is said to be Federal Trade Commission Chairman James C. Miller III.

The prominent role of Regan and the seemingly more distant role of Bush has produced criticism from others in the White House. "Regan doesn't realize that the people in Dubuque don't have the vaguest idea of who he is," one White House official said. "They think the vice president takes over" at such times. "They didn't vote for Regan."