WASHINGTON (AP) _ President Reagan has always been reluctant to fire aides when they're in trouble and he is unlikely to change that now by ditching Attorney General Edwin Meese III, presidential scholars and observers say.

Reagan didn't fire Labor Secretary Raymond Donovan, who was facing a federal court indictment in early 1985, nor did he oust Interior Secretary James Watt in 1983 after Watt made offensive remarks about blacks, women, Jews and the handicapped.

The closest Reagan came to being faced with a decision on what do about an indicted top aide was when Donovan was indicted. Donovan spared him the decision, however, by resigning immediately.

Watt also offered to step aside, and the only way Meese will leave is if he does likewise, several observers say.

Reagan so dislikes firing top aides that an embittered Secretary of State Alexander M. Haig Jr. had to try at least twice to quit before the president finally accepted his resignation in June 1982.

''After watching Ronald Reagan and reading about him all these years, I've come to the conclusion that it is a matter of leadership style that dictates that he not fire people,'' said Robert A. Bonatati, who was Reagan's labor adviser early in the administration.

''He never gets on the side of firing people and he always defends people who are under attack,'' Bonatati said of the president.

Reagan proved this theory once more Wednesday when besieged with questions about whether he would push Meese out in response to the attorney general's mounting legal problems and mass defections at the Justice Department.

''He's been a friend for over 20 years. I have every confidence in him,'' Reagan told reporters.

His statement recalled the time in the summer of 1984 when Reagan, arriving in Dallas for the Republican National Convention, was quizzed about whether he would oust Donovan. Reagan declared that he wouldn't give in to a ''lynch mob atmosphere.''

In quintessential Reagan style, the president threw his arm around Meese when reporters tried to question him about the attorney general early last fall, just after the two had appeared in the White House press room to announce Anthony M. Kennedy's selection for a Supreme Court vacancy.

Reagan has steadfastly said he thinks public officials ought to be presumed innocent until found guilty in court. He has gone even further in the case of indicted former White House aides John M. Poindexter and Oliver L. North, declaring that he thinks they are innocent - period.

While Reagan has been reluctant to fire people in general, he has shown even greater resistance when dealing with longtime confidants like Meese.

During his first months in the presidency in 1981, writes Reagan biographer Lou Cannon, ''Reagan valued Ed Meese's advice above all others, believing that Meese would see to it that conflicting points of view were presented to him without coloration.''

His close association with Meese goes back to Reagan's days as governor of California from 1967-75, when Meese was a valued counselor.

A former high-ranking official in the Carter administration, describing the difficulty that President Jimmy Carter had in ousting budget director Bert Lance, said ''it's always difficult for presidents to let go of people close to them, and that was true of Lance, and it's certainly true of Meese.

''These are the toughest issues for presidents to face,'' said this former high-level Carter associate, talking on grounds of anonymity.

Two presidential scholars said that, in his reluctance to get rid of aides who have become political liabilities, Reagan's style has differed from other presidents who have been faced with aides who became heavy political liabilities.

They pointed out that Dwight D. Eisenhower ousted top aide Sherman Adams and that President Lyndon B. Johnson got rid of close associate Walter Jenkins, while Gerald R. Ford eased out Agriculture Secretary Earl Butz.

''It's a major problem for all presidents,'' said the University of Virginia's Larry Sabato. ''But few presidents have been as reluctant and as reticent as this president to ditch people who become political liabilities. Eisenhower thought it was important to be cleaner than a hound's tooth.

''He may be completely innocent,'' Sabato said of Meese, ''but the truth is, he is costing Reagan politically, and he's certainly costing (Vice President George) Bush.''

James L. Sundquist, a senior presidential scholar with the liberal Brookings Institution, said ''politicians by nature tend to be very loyal and warm people, and they find it more difficult than crusty businessmen to fire people.''

''The responsibility really weighs heavily on the subordinates themselves, to recognize when they have become a political liability,'' Sundquist said. ''It's not a confession of guilt. He could still do what Donovan did, and get his name cleared.''

Bonatati noted he lost his job at the White House despite Reagan's aversion to firing.

He pointed out that when Faith Ryan Whittlesey arrived in early March 1983 to succeed Elizabeth Dole as director of the White House Office of Public Liaison, Whittlesey fired him and several other presidential advisers with short notice.

''She pretty much cleaned house,'' Bonatati said.