The Senate Foreign Relations Committee, after a sharp procedural battle that appeared to end in a defeat for the Republican majority, agreed yesterday to seek by subpoena, if necessary, the index to still secret tape recordings of White House conversations in 1973 between Alexander M. Haig Jr. and President Nixon.

Haig, who is President-elect Ronald Reagan's nominee for secretary of state and who served as White House chief of staff in the final months of the Nixon administration, sat silently at the witness table watching Republican and Democratic committee members argue over the subpoena issue for 90 minutes -- nearly half of his second day of confirmation hearings.

Haig has said repeatedly that he is not opposed to any subpoena action and that he served Nixon both loyally and legally. The committee's action, however, represents a potentially dramatic and, for many Republicans, unexpected and unwelcome turn in the confirmation process.

Under the complex and still not completely clear compromise wowrked out by the committee, yesterday's action could mean, for example, that controversy and an investigation into Haig's past could continue beyond the time when Haig is confirmed by the Senate and even beyond Jan. 20, Inauguration Day. h

The subpoena for the index and logs of conversations, if issued by the committee, is only the first step in what would be a longer process to listen to the tape recordings. It is also a process likely to be challenged in court by Nixon, or others that would cause further delays or rule out any committee access.

Sen. Howard H. Baker Jr. (r-Tenn.), the Senate majority leader and a committee member, said yesterday he expected to move in executive session of the committee this week that Haig be confirmed and that, as part of the compromise, the panel could look over whatever material may eventually be made available after the confirmation. Baker said there was a precedent for continuing a probe after a confirmation, an apparent reference to a Foreign Relations Committee investigation of secretary of state Henry A. Kissinger in 1973.

The battle over the subpoena issue, in the hearing room and behind the scenes, may also have put a dent in Baker's new role as Republican leader since he seemed to be the clearest loser in this first test of GOP solidarity.

Behind the scenes, sources say at least one Republican, Sen. Larry Pressler of South Dakota, said in a caucus that he would be "more comfortable" with a subpoena as a way to help clear the air about Haig. Sen. Charles McCMathias Jr. (R-Md.) also reportedly told colleagues he was prepared to vote for a subpoena. Thus, the Republicans, who have a 9-to-8 majority on the committee, might have lost a vote if they had decided to try and thwart the Democratic demand for such action.

Sen. Charles Percy (R-Ill.), committee chairman, also appeared to reject initial attempts by Baker to derail the subpoena request in the public hearings.

Under the compromise eventually worked out by Percy, with Baker's help, the committee will seek the quickest possible answer from the National Archives, where the tapes are stored, as to whether the index and logs can be obtained without a subpoena. If they can, Percy will write a letter requesting them and the Democratic and Republican counsels to the committee, and any senators that want to, can look over the logs to determine which might be relevant to Haig's suitability for the State Department job and then subpoena them. If the logs cannot be gotten by letter, Percy said the committee would issue an immediate subpoena for them.

Last night, committee staffers said they were still trying to work something out with the archives.

Either way, the committee has left open the question of who actually will have final say on determining what tapes should be subpoened as "relevant" if matters get that far. Percy, however, gave assurrances that anything determined to be relevant would be subpoened.

The Democrats requested logs to about 100 hours of White House conversations between May 4 and July 18, 1973. Sen. Claiborne Pell (D-R.I.), who made the subpoena request for the minority, argued that it was a considerable reduction from the amount of tapes and documents previously suggested and that if there is material on the tapes "to disqualify" Haig "then he should be. But on the other hand if there is not, then we'll all feel more comfortable."

Baker said he "had never been enthusiastic about any subpoena" and argued, along with other Republican members, that such action "will trigger things beyond our control," not just objections by Nixon but possibly court cases going on for weeks, months or years. He said Haig was the best witness and that an extended investigation could leave the new administration without a secretary of state "for months at a dangerous time in history and the world."

Sen. S. I. Hayakawa (R.-Calif.), argued that "the longer we take, the more we detract" from Haig's authority in foreign affairs and hurt the nominee's international standing.

Baker, early in the debate, said, "I'm determined not to vote for a subpoena for anything" until the committee goes through a process of determining what was relevant. In his opinion, Baker said, the committee was talking about "a fishing expedition for 100 hours of tapes in which we sit around and gorge ourselves on it."

Baker said it was "harder to give up on this issue than any I ever saw."

There could be some irony in this for Baker, and for the Reagan administration, if the dispute evolves into a real problem for Haig. When the former general's name was first mentioned as the leading candidate for the top State job, Baker reportedly privately warned Reagan that there could be trouble in the confirmation process -- and image problems for the Republicans -- if Reagan went back to such a controversial figure.

Later, after his own investigation, Baker said publicly he was convinced Haig had no skeletons in his closet, and he became one of Haig's strongest supporters.

Another possible irony is that the subpoena debate may mean that most public questioning of Haig about his past, rather than foreign policy, may be over, and whatever questioning evolves from any subpoenaed documents will be done in a closed-door session.

But at the moment, Haig's confirmation seems assured, and is likely before the inauguration.