Alexander M. Haig Jr., Ronald Reagan's choice for secretary of state, yesterday stoutly defended his performance in the service of another president, Richard M. Nixon, while Democratic senators for the first time called openly for authority to subpoena documents and tapes relating to Haig's White House years.

Nixon, Haig told the Senate Foreign Relations Committee on the opening day of his confirmation hearings, "was entitled to the presumption of innocence until proven otherwise." As for his own actions during those years, Haig said he "worked hard within the boundaries of the law and the advice of lawyers to support him."

Anticipating that the Democratic Senate minority would try and use his controversial role in the Nixon White House as a way to question his suitability for the nation's top diplomatic post, Haig came to the packed hearing room prepared to confront his critics head-on.

He produced a nine-page summary of what he called "the facts" on all the controversial items raised by the Democrats, including his roles in Nixon's pardon, wiretapping of officials and reporters and the bombing of Cambodia. He also told the committee that he, personally, had no objection to any subpoena and that he did not have anything to hide.

The former four-star general and commander of NATO troops won considerable praise from both Democrats and Republicans during the all-day session for the overall talents he has displayed through much of his 35 years of government service.

Nevertheless, a number of Democrats made clear that their concerns were not limited to specific acts that many or may not be recorded on some secret tape or stored in a locked file. Rather, a number of them focused on Haig's attitudes toward power, the presidency and the Constitution and questioned how those views, combined with his many years of military service, would influence his actions as secretary of state.

Before Haig said a word at yesterday's session, Sen. Claiborne Pell (D-R.I.), the ranking minority member on the 17-member panel, told the audience that "in the 16 years I have served on this committee, I do not recall a nomination that has come before us that has caused the concern and worry in the Senate that this one has."

Pell, in his opening statement, also quickly brought into focus the committee's sharp partisan split when he asked it to approve the issuance of subpoenas for specific records "to help judge what kind of a secretary of state" Haig would make. Pell and other Democrats argued that clearing the air would be to everybody's benefit, including Haig's, and the the symbolism of such a move would convey a true spirit of openness.

The committee chairman, Republican Sen. Charles Percy of Illinois, and the Senate majority leader, Sen. Howard H. Baker Jr. of Tennessee, warned, however, that such a move would probably lead nowhere, that Nixon would probably take legal action to stop it, and that it would delay confirming Haig, possibly until after the inauguration, which, they said, would be dangerous considering international troubles.

At the end of the witness table where Haig sat was a large pile of books and documents, apparently meant to be used by Republicans in his support if necessary. Percy referred to how much material was already available and, later in the hearing, Sen. Joseph R. Biden Jr. (D-Del.) pointed to the stack and noted its "symbolism," an allusion to Nixon's nationally televised defense of Watergate in which he surrounded himself with stacks of books containing transcripts from the White House tapes.

Percy temporarily resolved the tapes and documents dispute by asking committee counsels from both parties to meet and report by this morning on what courses seem practical to obtain additional documents that are not restricted by existing laws. Percy said he would go along with a request for subpoena power "if necessary."

In his prepared statement, Haig said the decision to use wiretaps to stop news leaks was the president's, not his, and that, when asked who had access to various information, he supplied names but "never decided on which individuals were to be tapped."

Haig also claimed that "at no time did I ever suggest in any way an agreement or 'deal' that Mr. Nixon would resign in exchange for a pardon from Mr. Ford."

He also sought to put to rest assertions that one tape of a conversation between him and Nixon on June 4, 1973, suggests that he suggested to Nixon that the president "should dissemble or pretend not to recall something" as part of his Watergate defense. Haig says he cannot recall all of the conversation, and that the tape is largely unintelligible, but that he thinks he was advising Nixon that, for his own sake, he should continue listening to the tapes since Nixon could not independently recall what had been said on various occasions.

Haig claimed that "although Watergate was obviously important" during his 17 months as White House chief of staff, "I spent 90 percent of my time trying to assure that the other business of the presidency was properly conducted."

Haig also addressed the covert Nixon-era activities in Chile aimed at preventing the elected Marxist president, Salvador Allende, from taking and exercising power. Haig claimed he "was not deeply involved" in policies toward that country and that "in general" he had no responsibility to review or approve any CIA covert activities in Chile.

It was on Chile, however, that several senators raised what seemed to be the most pertinent questions about Haig's past. Sen. John Glenn (D-Ohio), while generally supporting Haig's views on other issues, said it was Haig's role in the Chilean affair that kept coming back to his mind and causing him concern. Glenn wondered whether Haig might harbor instincts for bypassing legally constituted authority in dealing covertly with another country.

Sen. Paul E. Tsongas (D-Mass.) asked Haig if Reagan will bring the nation back to the days when it would seek to overthrow a government, and repeatedly asked if Haig thought the U.S. actions against Allende were proper. Haig said he wasn't going to be drawn into that and, in the one moment when he appeared to drop his outward calm and become combative, the former general referred Tsongas again to his prepared statement "since you are so interested."

In his opening statement, Haig told how he believed "passionately in the office of the presidency" and how he viewed his "overriding duty as one to preserve that office in the national interest."

Sen. Paul S. Sarbanes (D-Md.) asked Haig if that meant he felt that preserving that office came ahead of preserving the Constitution. "Not at all," Haig answered. "They are intimately interrelated in every sense of the word."

"I have witnesses two, three or maybe even four president at reasonably close range," Haig added, "and the point I'm trying to make is that the office itself, the institution of the presidency, is an extremely important safeguard for our people." That office, he said, "has led less than perhaps perfect men constantly to feel the weight of that office itself and to inevitably make judgments in terms of how they are going to be judged by history."

Sarbanes, noting that Haig briefly considered running for the presidency, also asked if it could now be safely assumed that Haig had given up such ambitions and would not carry that as an added complication in the State Department.

"I can assure you," Haig answered, that his acceptance of Reagan's invitation to take the post "was made exclusively, in fact in spite of, any other political ambitions or hopes I might have held for myself."

And what was the obligation of officials in the executive branch to follow orders of the president, Sarbanes then asked.

Someone who received an order that is morally wrong or consistently against his thinking, Haig said, should not obey it and should also separate himself from the one giving the orders. It's a very personal thing, however, Haig added,something that bureaucrats throughout the government face every day. In response to a question, Haig said orders generating an illegality should not be obeyed. Was former attorney general Elliot Richardson then not correct, Sarbanes asked, in refusing to fire Watergate special prosecutor Archibald Cox? That, Haig said, is a difficult question to answer and was a decision made by lawyers at the time.

Haig won praise from committee members for volunteering to testify under oath and Sen. Richard G. Lugar (R-Ind.) read additional praise for Haig into the committee proceedings in the form of quotes on Haig's performance from former Watergate prosecutor Leon Jaworski.

Questioned by Lugar about his finances, Haig said that taking the State Department post meant giving up perhaps $8 million to $9 million in salary and other benefits over the next four years that would have been his had he stayed as president of United Technologies Inc.

Why would anyone give this up after 35 years of government salaries, Lugar asked.

"It is a reflection of my concern about this country and its recent drift," Haig answered. He was not speaking in partisan terms, he said, because the confusion has been going on for 15 to 20 years. "I suppose we all have only one life to give and for me, my reward would come from being hopefully successful at putting some greater clarity, sense of direction and some greater effectiveness into American foreign policy. That's the greatest reward I could leave my children."