If members of the Senate were voting today on the new SALT II agreement and all 100 remained true to their present emotions, the vote probably would be about 40 in favor, 20 against and 40 abstentions.

Those abstentions would be the undecided senators who eventually will determine whether this new package of agreements with the Soviet Union will be approved by the necessary two-thirds Senate majority. Happily for those 40, a final vote is not likely for about five months at the earliest.

Predicting the final outcome now would be rather like predicting next October's weather. So many unforeseeable factors are likely to affect the Senate vote that no handicapper can look ahead with any confidence.

The Carter administration's most knowledgeable lobbyists acknowledge that winning the votes of 67 senators will be difficult. SALT opponents predict confidently that 34 senators eventually will vote no, or that 51 will vote for substantial changes in the agreements.

Some senators speculate that the governing factor may be the administration's decisions on new weapons that would be permitted even under SALT II. According to this school of thought, Carter can "buy" the Senate votes he needs by approving certain military programs, particularly full-scale development of a new missile called MX that would be land based in a way that could ensure its survival against a Soviet attack.

But a small bank of liberal senators ardently oppose the MX missile, and several have threatened to vote against SALT II if they are forced to take MX along with it.

The handiest precedent for the SALT II debate is last year's dramatic struggle over the Panama Canal treaties, which the Senate eventually approved with just one vote to spare. According to some analyses - usually offered by critics of SALT II- at least a dozen senators who supported the Panama treaties have indicated skepticism about SALT and so it probably faces defeat.

But the relevance of the Panama precedent is debatable. A Senate aide favorably disposed to SALT noted, for example, that many senators suffered politically by voting for the Panama treaties, which were never popular. Arms control, on the other hand, enjoys great public support, at least in principle, this aide said.

The administration hopes to prove that SALT and Panama are not comparable. Many administration officials contend that senators will be intimidated by the ultimate prospect of accepting responsibility for defeating SALT II.

Indeed, administration officials and their allies in the Senate think their best weapon in the debate will be senators' reluctance to invite even the possibility of renewed cold war and accelerated arms race.

Optimism that the Senate will eventually find the treaty irresistible assumes an effective selling job by the administration, however. Many senators, including supporters of SALT, privately question whether this administration can rise to the occasion.

Some of the most ardent supporters have been complaining for nearly two years that the administration has failed to argue effectively for the arms agreement. Many express the belief that the critics of SALT have controlled the debate thus far.

But there are also signs of optimism among SALT supporters that at last the administration has found a way to argue for the treaty. The key, a Senate aide said this week, is "SALT without detente"-a defense of the agreements strictly on their merits as enhancements of national security, without regard to whatever the Soviet Union may do on other issues.

Critics of SALT claim to be content with this approach too. They contend the agreements are obviously disadvantageous to the United States because they allow too many potential or actual advantages to the Soviet Union.

At the White House, the Pentagon and the State department, officials insist that they could not actively promote SALT II until it was completed, but that now they are ready to press their case in the country and in the Senate.

The White House has organized a national campaign, and senior officials promise it will be the most effective public relations effort of the Carter administration.

Administration lobbyists say Defense Secretary Harold Brown will prove to be their secret weapon. His good reputation and his authoritative mastery of the SALT issues will convince skeptical senators, they predict.

Inside the Senate, four men are likely to play disproportionate roles in the debate. Both the Carter administration and the opponents of the agreement will concentrate their attention on this small group.

The first is Robert C. Byrd (D-W.Va.), the majority leader. By instinct Byrd is a hardliner on defense issues. But he sees himself as a statesman and has been loyal to President Carter on almost every issue. Administration officials acknowledge that, without Byrd's support, SALT II would be in dire danger.

The second is Howard H. Baker Jr. (R-Tenn.), the minority leader and an increasingly active candidate for president. For the record now Baker says he is "leaning against" SALT II. But he has told important figures in his home state that he would like to be able to vote for it. Colleagues see Baker in a dilemma: to please delegates to the Republican National Convention-a conservative group-he should probably oppose SALT, but to follow his true instincts and also be electable to the White House, he might prefer to support the agreements.

One option that appeals to Baker would be a Senate vote to return the treaty to President Carter for further negotiation to satisfy the concerns of skeptical senators.

The third key senator is John C. Stennis (D-Miss), 77-year-old chairman of the Armed Services Committee. Stennis commands enormous respect and influence in the Senate. He supported the Vladivostok accords of 1975 which are the basis for the SALT II agreement, and the administration hopes he will eventually come out for the new pact. Stennis is one of the senators who is likely to be influenced by administration policies on other arms programs.

So will Sam Nunn (D-Ga.), the fourth key senator in the SALT debate. Nunn has been cultivating a reputation as a moderate, pragmatic expert on military affairs, and he enjoys the respect of many colleagues. He believes the United States has failed to respond adequately to the buildup of Soviet military forces, and he has asked already for an expensive list of new military programs to compensate for Soviet improvements. Nunn has suggested that SALT II itselt may be less important than the rest of the U.S. defense program.

Nunn, it is thought, could influence Stennis' final position, and many moderate Democrats and Republicans say they are looking to the Georgian to help thme make their own decisions on SALT II.

Thus far, three broad areas of concern have emerged as potentially decisive in the Senate debate: the adequacy of American defenses outside SALT II, the fairness or equality of the balance of strategic forces permitted by SALT II, and the ability of the United States to verify Soviet compliance with the agreements.

In the verification field, a fifth senator has sought to establish a special role for himself. John Glenn (D-Ohio) is unhappy about the means the administration envisages for checking up on Soviet compliance, and many colleagues grant the former astronaut special expertise on this. As another senator said recently, "He knows what it looks from space"-a reference to the space satellites that are the most important single source of verification information.

Colleagues who think they know Glenn well predict that he will eventually come down on the side of SALT II, but this is far from certain. Administration officials think they have a potent verification argument, which runs, briefly, like this: if senators are uncomfortable without the ability to verifty a treaty that requires both sides not to conceal their strategic programs, how will they feel about verifying Soviet programs in the absence of such a treaty?

It the administration does win the support of four of these five senators-all but Baker seem possible, perhaps likely, and Baker is not ruled out-the Senate's Republicans, particularly moderates, may hold the balance of influence.

In such a case, ironically, the key figure might turn out to be a man who is deeply steeped in SALT, but who is not a member of the Senate. As a Senate aide said this week, "Henry Kissinger can give any Republican senator an excuse to vote for SALT, if that's what they're looking for." CAPTION: Picture 1, ROBERT C. BYRD, Picture 2, HOWARD H. BAKER JR., Picture 3, JOHN C. STENNIS, Picture4, SAM NUNN, . . . these senators are likely to play key roles in debate over strategic arms limitation treaty with Soviets.