Questions about former general Alexander M. Haig Jr.'s assistance to Richrd M. Nixon in the final stages of the watergate affair have put his appointment as secretary of state in jeopardy, authoritative sources said yesterday.

Sources close to President-elect Ronald Reagan said that the president-elect and his associates were concerned by the warnings they had received from Sen. Howard H. Baker Jr. (R.-Tenn.), the future Senate majority leader, that Haig could run into trouble in the Senate. This has put Haig's nomination in jeopardy, these sources said, though many around Reagan still want him for the job.

Haig's cause is also being hurt by the fact that Nixon himself is campaigning avidly for Haig's nomination to be secretary of state, knowledgable sources said.

If Reagan decides he cannot select Haig, sources said, he will turn to George P. Shult , Caspar W. Weinberger or William J. Casey to fill the State Department job. Shultz has told Reagan he does not want the appointment, but sources close to Reagan hope he might reconsider.

Weinberger is in line to become secretary of defense and Casey director of the Central Intelligence Agency in the Reagan administration. These posts might be rejuggled if Haig is dropped as secretary of state.

According to authoritative sources, Reagan's associates only recently asked Republican leaders in the Senate if they thought Haig would run into difficulty in the confirmation process were Reagan to nominate him for secretary of state. The answer was discreet but clearly cautionary: study the public record on Haig's Watergate role very carefully before proceeding. Apparently, the leadership drew special attention to Haig's role in the pardon granted to Nixon by his successor, Gerald R. Ford.

According to these sources, the Republican leadership thinks Senate Democrats might decide to draw Haig through the Watergate mud if he is proposed for State. It's also possible that the Democrats will give President Reagan a honey-moon and not challenge his Cabinet nominees in the confirmation process. "Frankly we don't know" what they'll do, said one source on Capitol Hill.

The Democrats who will decide how much of a fight to make against Reagan's nominees indicated yesterday that they have not decided either. But anti-Haig sentiments are strong among Senate Democrats. Of the names mentioned as possible members of the Reagan Cabinet so far, one well-placed Democratic senator said yesterday, "Haig is the best candidate . . . to be looked at closely."

Haig's status in the increasingly mysterious Cabinet selection process could not be clarified yesterday, but sources close to the former White House chief of staff and North Atlantic Treaty Organization commander said he still had heard nothing from Reagan. These sources, who emphasized that they had no inside information, said that if, in the world of business, something had been under consideration for so long without any decision being taken, it would probably be considered "a dead issue."

Haig was in Washington yesterday. Apparently he is being asked to make the case for himself to Reagan aides who are trying to evaluate the dangers of going ahead with his nomination.

In 1974, it was rumored that Ford offered Haig the job of Army chief of staff, a post that requires confirmation by the Senate. Instead Haig took the job of NATO commander in Europe, which did not require action by the Senate. At that time, John C. Stennis (D-Miss.), chairman of the Senate Armed Services Committee, said he would hold a hearing on the Haig appointment to NATO even though this was not required by law, but later Stennis changed his mind and no hearing was held.

If Democrats on the Senate Foreign Relations Committee decided to make an issue of Haig's role in the Nixon administration in a confirmation hearing, they could be expected to raise the following episodes:

The so-called Nixon wiretaps, which the FBI put on the telephones of 17 reporters and government officials in 1969-71. FBI records list Haig as the official who requested the wiretaps in 12 of the 17 cases.

Haig testified in 1974 before the Foreign Relations Committee that he had received four of the names of people to be tapped directly from Nixon, and "all other names that I ever conveyed were names given to me by Henry," a reference to his former boss in the Nixon White House, Henry A. Kissinger. But when asked about specific cases like that of now New York Times columnist William Safire, Haig swore he had not asked for a wiretap, though the FBI records said that he had.

Haig's role in efforts by the Nixon White House to obstruct Watergate investigations, and later to withold tape recordings of Nixon's Oval Office conversations from special prosecutor Leon Jaworski.

On at least two occasions Haig attempted to get the attorney general's office to intervene with special prosecutors Archibald Cox and Leon Jaworski to try to restrict their investigations. Haig delayed the Senate Watergate committee's investigation by refusing to testify before it on the grounds that his communications with the president were privileged because of the "attorney-client relationship." This plea was rejected by the Justice Department, which pointed out that this privilege did not cover persons playing an intermediary role, like Haig, but only applied to lawyers and their clients.

According to Jaworski's memoirs, Haig actively participated in the effort to withhold the tapes that eventually led to Nixon's downfall. At one point, Haig warned the special prosecutor, "Things are going to get bloody, Leon," apparently hoping to dissuade him from pursuing the investigation too aggressively. Jaworski concluded that one of Haig's jobs as White House chief of staff "was to try to placate me while helping Nixon frustrate me in my efforts to move forward in the search for truth."

Senators could also question Haig about his various explanations of Watergate-related episodes:

The 18 1/2-minute gap in a Watergate tape. Testifying at a court hearing into the never-explained gap in the key Nixon tape, Haig said it might have been caused by an unexplained "sinister force" that had "come in and applied [another] energy source and taken care of the information on that tape." On the day Nixon announced his resignation, Haig had another explanation. According to Jaworski's memoirs, Haig said: "I haven't the slightest doubt that the tapes were screwed with. The ones with gaps and other problems."

Haig's role in the pardon of Nixon by Ford. According to Ford's memoirs, Haig memoirs, Haig raised the idea of a pardon in a conversation with Ford on the afternoon of Aug. 1, 1974, in a way that allowed Haig to go back to Nixon and say he had raised the matter, and that Ford had not shown any reluctance to consider it.

Ford's closest aide, Robert T. Hartmann, has described his anger when Ford told him about this. Hartmann said in his memoirs that he thought Haig had committed a "monstrous impropriety" by "mentioning the word 'pardon' in his [Ford's] presence," especially when the two men were alone -- at Haig's insistence. According to the memoirs, Hartmann and other Ford aides thought Haig had boxed Ford into an impossible position.

Advice Haig gave to Nixon and Kissinger on controversial policies during the Vietnam war. For example, if Haig were asked about his recommendation that the United States conduct a punitive air strikes against Hanoi at Christmas 1972, he could be put in the difficult position of either describing in detail his role in that controversial episode or claiming executive privilege for past conversations with a executive privilege for past conversations with a president and his national security adviser. Either option could prove embarrassing to Haig.

Some Republicans fear that confirmation hearings for Haig could reopen both Watergate and the Vietnam war, two painful memories from past Republican administrations that the Reagan administration had nothing to do with. There's no need to taint Reagan with this unfortunate history by putting Haig in the new administration, these Republicans contend.