Republican members of the Senate Foreign Relations Committee, which will hold confirmation hearings next month on the nomination of Alexander M. Haig Jr. as secretary of state, have hired Fred D. Thompson, former Republic counsel to the Senate committee that investigated Watergate in 1973, to help during the Haig hearings, according to transition officials.

Democrats on the confirmation panel have made it clear that they intend to delve heavily into Haig's role in the Nixon White House, and have voiced their intention to hire Terry Lenzner, who was the assistant chief counsel for the Democratic majority on the Watergate committee.

In effect, the hiring of Thompson, which reportedly was handled by the incoming Senate majority leader, Howard H. Baker Jr. (R-Tenn.), is meant to balance Lenzner's presence at the confirmation hearings scheduled to start Jan. 9.

One senior official on the transition team of President-elect Ronald Reagan, dismayed over the 7-year-old guns of Watergate being rolled out, said, "It is like my whole life flashing before my eyes again."

Though Watergate, and Haig's role during the final stages of the Nixon administration, is certain to be explored during these hearings, sources close to the Democratic minority say they intend to focus prinicipally on Haig's earlier four years of service as deputy to Henry A. Kissinger when he was White House national security affairs adviser. It was then that a number of controversial national security decisions were made, including Vietnam war strategy and wiretapping of various government officials and journalists.

Democrats on the staff of the Foreign Relations Committee have made preliminary requests of the National Archives for a copy of the index of presidential tape recordings made in the Nixon White House. The archives is in charge of some 6,000 hours of Nixon tapes, and the request by Senate Democrats could indicate that they will ask for many of the tapes not yet made public involving Haig's conversations in the Oval Office.

Sen. Charles H. Percy (R-Ill.), the soon-to-be committee chairman, who has made it clear that he does not want the hearings to become a second Watergate, is cool to the idea of reviewing the tapes, and said through a spokesman yesterday that he had no plans "for the present" to seek them.

Haig had a one-hour luncheon on Capitol Hill yesterday with Percy, Baker and Sen. Paul D. Laxalt (R-Nev.), which transition officials said was, in effect, the beginning of the confirmation process.

A number of top Republicans on the Reagan transition team say they believe the Democrats on the panel may be "going bananas" over the upcoming hearings in an effort to get Haig or embarrass Reagan through the Nixon-Watergate connection. Many Republicans are expressing the view that this could backfire on the Democrats because public opinion will rally to the support of Haig if he is seen as being harrassed without any new and harmful evidence being developed.

Fred Ikle, director of policy on the Reagan foreign policy board, told reporters yesterday that he thought Haig will be confirmed with relatively few dissenting votes and in a relatively short confirmation process.

Ikle said attempts to thwart the Haig nomination would "peter out" because public opinion would "rally behind Haig and the new president" at a time of crisis in foreign policy. "The public reaction, I predict, will be that 'the time for silly games is over and let's get on with the show,'" meaning the Reagan administration.

Some Republican officials predicted that senators will hear from their constituents via mail if they spend too much time reviewing the Nixon years during the Haig confirmation.