Four weeks of hearings on the new strategic arms limitation treaty appear to have enhanced prospects for eventual approval of SALT II, in the view of many in the Senate on both sides of the issue. But the treaty still needs 67 votes, and its supporters concede they still cannot see exactly where they will come from.

A month ago, the treay's supporters anticipated the hearings with some dread, but in fact the intervening time has brought a number of surpirses that all seem to help their cause. Issues that once looked ominous have been defused, and the debate about the merits of the treaty document has been largely supplanted by a debate over American defense spending.

Even some of the treaty's opponents acknowledge that the Senate now appears much more likely to grant its approval of SALT II.

"I smell ratification," one of the Senate's best head-counters observed and this is a conservatie who probably will vote against the treaty. "I wouldn't have said that five or six weeks ago," he added.

In the highest reaches of the Carter administration, these first weeks of hearings have produced a sense of relief and some optimism. "We still have to get 67 votes," one senior official observed, nothing that the basic political challenge posed by the SALT ratification fight remains to be met. But like others, this man believes that the treaty is in far better shape today than it was a month ago.

Just five weeks ago, Minority Leader Howard H. Baker Jr. (R-Team.) announced his strong opposition to SALT II as submitted to the Senate. Baker's statement alarmed the White House and raised doubts about the prospects for winning two-thirds support for the treaty.

Baker was seen as a key figure five weeks ago, but today in the Senate his star seems to be on the wane, at least in terms of the SALT debate. That is one of the changes that have occurred during the Senate hearings.

Baker tied his opposition to specific treaty provisions, particularly the one that would allow the Soviets to maintain 308 "heavy" supermissiles that the United States could not match in its arsenal.

But when the hearings began Baker had trouble arguing his case against the treaty. He was sharply rebutted by administration witnesses and felow senators, and soon retired from active participation in the Foreign Relations Committee's hearings. The leadership role he had hoped to play did not materialize.

A month ago "verification" looked like a potentially decisive issue. The word refers to America's ability to monitor Soviet compliance with the detailed provisions of the treaty, and for a time it looked as though the recents loss of spy stations in Iran might lead many senators to conclude that the treaty could not be monitoried.

Instead, Sen. Barry Goldwater (R-Ariz.) has stated publicly that he thinks the verification problems are manageable. Adm. Stansfield Turner of the CIA has hardened his support for verifiability of the treaty. The issue still concerns some senators, but now seems unlikely to determine many -- if any -- votes.

Opponents of SALT II were saying privately in June that, when the hearings began, numerous new loopholes and shortcomings in the treaty would be revealed.But this hasn't happened.

Instead the critics have complained about alleged inequities in SALT II that have been public knowledge for montqs. Their very familiarity may have detracted from their impact in the hearings.

The Carter administration feared that Sen. Henry M. Jackson (D-Wash.), the dominant senator in the 1972 debate on the first SALT agreements, would have a bag-full of embarrassing surprises when the Armed Sevices Committee began hearings last month. Instead Jackson's points were familiar and predicatable, to the administration's relief.

In private conversations this week, senators of many different persuator has worked to SALT II's advantage the total absence of public interest in the proceedings.

"The lack of public interest is unbelievable," added a conservative Republican from the West. And both of them agreed that public indifference tends to help the treaty's prospects.

"This isn't like Panama at all," one senator said, a comparison to the intense public sentiment that developed in many states against the Panama Canal treaties. Once the SALT II debate looked like something a replay of the Panama fight; now the parallel seems less clear.

The emergence of the military spending issue last week changed the nature of the SALT II debate, and probably transformed it. Sen. Sam Nunn (D-Ga.) set the new course, and Henry A. Kissinger tacked his personal ship-of-state in the same direction. The specific terms of the treaty are no longer in dispute for those who have taken this line; instead the question is whether the United States will pursue an aggressive defense buildup to match the Soviets in time for SALT III.

The sudden prpularity of this approach has created a new political problem for the Carter administration, but it is one the White House seems to prefer to a knockdown battie over the technical provisions of SALT II. The problem is to find a fermula that looks like a real increase in defense spending to hawks, but not an abomination to doves. This won't be easy.

Sen. George McGovern (D-S.D.) predicted this week that the sudden new consensus that seems to be building behind increased spending for defense will probably leave him behind -- the treaty is now likely to pass. McGovern said, "without my vote." In other words, one of the leading Senate advocates of arms control and lower defense spending sees his colleagues running off in this new direction in such large numbers that votes like his won't be needed to approve the treaty.

The White House isn't yet sure it can count the 67 senators who would be on such a new team of treaty supporters. Nor is the administration yet certain how it can ask for more defense spending in a tight election-year budget when a recession is clearly coming. But despite the technical and political difficulties, senior officials are feeling a lot better about the prospects for SALT II than they were a month ago.

It may be possible to build a Senate consensus behind both increased defense spending now and some from statement in conjuction with SALT II that SALT II than they were a month ago.

It may be possible to build a Senate consensus behind both increased defense spending now and some firm statement in conjuction with SALT II that SALT III must include substantial reductions in levels of arms to be acceptable to the Senate.

Sen. Alan Cranston (D-Calif.), the majority whip, predicted that this combination will prevail. CAPTION: Picture, Alexander Haig: He would refuse to vote on SALT if he were a senator. AP