Zbigniew Brzezinski, President Carter's national security adviser, has turned aside a request by Senate Foreign Relations Committee Democrats for assistance in securing access to former president Nixon's tapes and other materials relating to Alexander M. Haig Jr., Ronald Reagan's choice as secretary of state, according to sources on Capitol Hill.

On Christmas Eve, Brzezinski sent a letter to Sen. Claiborne Pell (D-R.I.), who will be the ranking minority member of the committee in the next Congress, outlining why he could not assist the Democrats. The two-page letter said all of the materials sought would likely be subject to executive privilege invoked by Presidents Carter, Ford and Nixon.

Even more troubling to the Senate Democrats is a reference in the letter to waiting for an additional request from the full committee which, beginning Monday, will have a Republican majority, before the White House would even begin processing the materials.

Brzezinski, who said Sunday on "Meet the Press" that Haig is qualified to be secretary of state, had reportedly told White House aides that he favors Haig's appointment.He is also known to feel that normal procedures require that the White House wait for a full committee request before turning over any material -- and that even then Carter, Ford or Nixon will likely interpose an executive privilege plea.

A White House official said last night that in any case involving executive privilege, White House lawyers' would have to review the materials before they could be released.

Senate Democrats consider it highly unlikely that the Republican majority of the committee will go along with any such formal request.

Brzezinski's position and the reluctance of Senate Republicans to allow Haig's confirmation hearings to become a replay of Watergate have led Senate Democrats to lose hope for "any meaningful investigation" of the most controversial portion of Haig's extensive record of public service, these sources say.

Pell had been advised by Senate lawyers that the committee could only get the tapes and other materials from the Nixon period under two conditions -- Nixon's consent or Carter's judgment that release of the materials is "necessary for the ongoing operation of the executive branch."

Materials relating to Haig's service as commander of the North Atlantic Treaty Organization during the Carter administration could also be released at the discretion of Carter.

On Dec. 18, Pell sent general requests for materials to Nixon, as well as to the National Security Council and various other federal agencies. Nixon's lawyer, Herbert (Jack) Miller, responded almost immediately by asking for a more specific request.

Pell forwarded the specific request for any tapes and documents relevant to Haig's involvement in wiretapping and controversial foreign policy advice while serving as Henry A. Kissinger's deputy, his role in directing Nixon's Watergate and impeachment defenses, and his discussions with then-vice president Ford about a future pardon for Nixon.

Capitol Hill sources say, however, that Senate Democrats had believed it more likely that the Carter White House would provide the materials in timely fashion for the pending hearings -- which begin Jan. 9 -- than Nixon would agree.

While Brzezinski's letter left little hope for a positive reponse to the Democratic request, it did request that Pell "define with greater specificity the documents you believe relevant to the hearings." Pell was known to be unhappy that Brzezinski took a full week of the three weeks before hearings to ask for a more specific request.

Apparently concerned that Brzezinski might sidetrack his request, Pell directed his detailed response and a renewed plea for the materials to the president personally.

Even prior to Haig's formal designation by Reagan, the Democratic members of the committee sought to retain Terry F. Lenzner, the former Senate Watergate committee deputy chief counsel, to conduct a full investigation of Haig, including extensive interviews with Haig's former White House and National Security Council colleagues, as well as a review of the documentation sought by Pell.

Lenzner told Pell and incoming committee chairman Charles Percy (R-Ill.) that he was reluctant to take on the job unless the committee was willing to conduct a thorough, bipartisan investigation, according to Senate sources. Percy balked at the request, but after being reminded by his staff that he had been the ranking minority member of the Governmental Affairs Committee at the cursory confirmation hearings given Bert Lance, Carter's first director of the Office of Management and Budget, he relented. Lance later resigned following allegations of irregularities in his banking business. l

However, Lenzner was unable to implement such an arrangement with Fred Thompson, the Republican special counsel to the committee, reportedly selected by the next Senate majority leader, Howard H. Baker Jr. Thompson served as Baker's minority counsel during the Senate Watergate hearings.

The committee Democrats terminated discussions with Lenzner shortly after the Brzezinski response, since, sources say, they had given up hope for developing any new information relevant to the hearings.

Henry F. Schuelke III, a former assistant U.S. attorney here, was subsequently retained by the committee to assist in preparing questions for the Democratic senators.