Defending his chief of staff against the charge that he was responsible for firing Margaret M. Heckler from the Cabinet, President Reagan said yesterday of Donald T. Regan: "He's on our side."

But senior administration officials familiar with the sequence of events that led to Heckler's removal as secretary of health and human services said Regan has never sided with her and was determined to force her out of the Cabinet. A well-informed White House official said Regan sent Heckler "signals of nuclear proportions" that he wanted her to leave, adding, "she refused to hear the signals."

Since Regan left the Treasury Department early this year in a job switch with James A. Baker III, he has made no secret of his desire to put a corporate stamp on the presidential Cabinet and push out those whose style or competence he questioned. An official close to Regan said Heckler was always "the first to go" on Regan's list.

It is nothing new for a Reagan chief of staff to take aim at a member of the Cabinet. Baker openly called for the removal of Secretary of Labor Raymond J. Donovan when he came under investigation for alleged links to mob figures. Several White House officials, including Baker and then-counselor Edwin Meese III, participated in the conflicts which led to the resignation of Secretary of State Alexander M. Haig.

White House officials also played key roles in the resignations of Secretary of Interior James G. Watt and Anne M. Burford, head of the Environmental Protection Agency.

But in all of these resignations the president remained carefully distanced from the machinations of his aides. They never put him in the awkward public position of dismissing a Cabinet member, let alone of trying to represent a firing as something akin to a promotion. For a long succession of Reagan chiefs of staff, in Sacramento and in Washington, it was axiomatic that Reagan was uncomfortable with personnel decisions and that he "never fired anybody."

Heckler's case was different, both her supporters and critics say, because Regan's approach to his job as chief of staff is different from that of his predecessors. Under Regan, the White House has become hierarchical. The Regan team, as one official said proudly last week, "carries out his directions."

High among these directives was the ousting of Heckler, whom Regan has disliked since the days when they were equals in the Cabinet. A senior administration official said the White House chief of staff made the dismissal of Heckler an issue of his standing and credibility, adding that "he really drew the line."

During Reagan's first term, the conflicting power centers in the White House gave an embattled Cabinet official an opportunity to find an ally. Meese, for instance, rallied to the support of Donovan. Baker spoke up for Office of Management and Budget Director David A. Stockman and may have saved his job when Meese and others thought Stockman should be fired because of his published confessions of doubts about Reagan's economic program.

But in the second-term White House, where Regan is the undisputed strongman, there are no rival power centers that a Cabinet official in disfavor can use as a court of appeals.

Heckler, a 54-year-old former Republican House member from Massachusetts, has been considered too liberal for administration conservatives ever since she was appointed in 1983 partly to blunt charges of unfairness in administration policies dealing with the poor. But by most accounts, Regan's objections were almost entirely to Heckler's political manner and personal style rather than her ideology.

"She was very individualistic and wanted to deal with the president directly," said a White House official. "She didn't comport to Regan's idea of what a Cabinet secretary ought to be."

Regan associates said he arrived in the White House determined to replace her. His negative views may have been reinforced by John A. Svahn, the White House domestic policy chief who was Heckler's former deputy and who considered her a poor administrator. Whatever difficulties Heckler had in management -- and even her supporters acknowledge that this is not her strong point -- were compounded by the unwillingness of the White House to fill key vacancies in her agency, which further weakened its administrative performance.

Officials say that Regan's plans to replace Heckler during the summer were set back by the president's surgery and by an operation that Heckler underwent about the same time. She saw the hesitancy as weakness and fought back, telling lawmakers and reporters that she didn't want to become ambassador to Ireland and "worked for the president, not his staff."

This defiance increased Regan's determination to get rid of her, officials said.

The chief of staff prepared talking points for the president, one of which described the post of Irish ambassador as a "promotion." Reagan, who is proud of his Irish roots, agreed to make the argument and, according to one White House official, readily agreed to try to sell the job switch to a skeptical press corps.

"Reagan thinks Don Regan got a bum rap and wanted to go out and do it," the official said. In the atmosphere of the Regan White House, apparently no one tried to talk the president out of what some thought was an embarrassing performance.