For many months now, official Washington has accepted a scenario for the unfolding of the Great Debate about a strategic arms limitation treaty (SALT) between the United States and the Soviet Union:

The two countries would finish negotiating the treaty early in 1979. A Brezhnev-Carter summit meeting would formally launch it, the Senate Foreign Relations and Armed Services committees would hold hearings, then the Senate would conduct the debate and vote yea or nay-all this to transpire before the next great national debate, the 1980 presidential campaign.

But now there is every evidence that this clean progression of events no longer is in prospect. The chances of the SALT debate being resolved this year are 50-50 at best and declining. The chances of a single, winner-take-all vote for or against SALT and arms control have disappeared.

There seems no prospect of separating the SALT debate from 1980 presidential politics, which already are under way. The Senate now could not debate SALT before autumn, when the many Republican candidates for the White House will be galloping.

The White House has begun to adjust to this prospect, and many officials there are coming to the conclusion that it cannot hurt their president if SALT and related issues become an important factor in 1980. They welcome the prospect of running as proponents of peace, SALT and a larger defense budget, or so they say.

The prospects for the SALT debate have shifted during the last three months. Perhaps the last chance to get a neat and clean SALT debate in 1979 was missed in late December, when Secretary of State Cyrus R. Vance and Soviet Foreign Minister Andrei Gromyko failed in Geneva to wrap up an agreement.

Since then the nature of SALT considerations in the Senate has changed.

Many members have begun to study the treaty-whose broad outlines have been known for months-and a number of key members have begun to take public positions.

The most significant of them may be Howard H. Baker Jr. (R-Tenn.), the minority leader.Baker's support for the controversial Panama Canal treaties last year made possible their approval in the Senate. The Carter White House has held out high hopes that Baker would eventually come down in favor of SALTT II and bring along a few Republicans, too.

Baker has reportedly told influential Tennesseans in private that he would like to be able to vote for SALT II. but at the same time he has taken the public position that he is "leaning against" the treaty.

He reiterated that position yesterday at a breakfast meeting with reporters, adding pointed criticisms of the SALT agreement that the administration could not easily satisfy.

For example, Baker said, "I don't think there's any way they [in the Carter Administration] can legitimately claim" that SALT II is verifiable by U.S. spy satellites, radars and other technical means. That suggests Baker might demand changes in the treaty to make it easier for the United States to be sure the Soviets were adhering to it.

Under the terms of SALT II as negotiated thus far, a Soviet medium-to-long range bomber known as Backfire is not counted as a strategic weapon. Baker said yesterday, "I can't think of anything that would convince me that Backfire should be excluded" from SALT II. He could propose an amendment including it.

"Those points could cause serious problems," an administration official said yesterday when informed of Baker's statements.

The administration reacted similarly last weekend when Sen. John Glenn (D-Ohio) released a speech in which he said substantial changes had to be negotiated in the treaty to make it verifiable by the United States. The White House had been counting on Glenn as a potentially influential SALT supporter.

Earlier last week three Republicans whose names appeared on administration list of potential SALT supporters joined in signing a letter proposing changes to the treaty that the adminstration regards as unacceptable.

Even before these developments the White House had a difficult time counting 67 senators who might support SALT II. Though senior officials remained hopeful that public sentiment, the debate itself and other unforseeable factors would eventually bring a Senate victory, they have also acknowledged the possibility of defeat.

The administration has long insisted that SALT II could not be toyed with by frustrated senators. "Brezhnev is not Torrijos," administration officials have often said, a reference to the Panamanian strongman, Omar Torrijos, who did accept Senate amendments to the Panama Canal treaties. The Soviet president, Leonid I. Brezhnev, would not be so flexible, they argued.

Whether independent-minded senators would have accepted this argument has always been problematical, but evidence is accumulating that they will not accept it now. Baker said yesterday it was the Senate's consitutional duty to give both advice and conset, and he suggested four possible courses the Senate might follow:

Approve SALT II as submitted, reject it, amend it or return it to the president for further negotiations.

Baker makes clear his distate for rejection. That course is not politically attractive-no presidential candidate has come out for scuttling the entire SALT process-nor does it offer much comfort in the realm of strategic policy. Baker talks like a man who wants to be for SALT in general but against SALT II in particular, particularly while he is running for the Republican nomination for president.

Senators and Senate aides who are favorably disposed toward SALT II join administration officials in arguing that even if the treaty is delayed, and even if the Senate spends a lot of time trying to amend it, their cause could still prevail.

At the White House officials insisted that "we've got the best arguments on this one," and that a battle for SALT II led by Carter eventually would succeed.

During the long delay in completing SALT II, the Carter team has also begun to get its act together. If the treaty had been completed a few months ago, the administration's "salt selling" machinery would have been much less effective than it now will be, officials conceded.

Now, official sources insist, the final stages of the negotiations really are going well. "The smiles of Cyrus Vance and Anatoliy Dobrynin last Saturday were justified," one authoritative source said, referring to an apparently important negotiating session that day.

But even a speedy agreement and early summit will not allow for a Senate vote before fall, probably October or November, unless senators give up their August holiday, which is now written into law. A great deal could happen before fall. One much-discussed possibility is a sudden turn for the worse in Brezhnev's frail health.

Another possibility is that the opponents of SALT II-who include articulate and foreceful proponents of a harder line in American policy-will continued to dominate the public debate on the treaty as they have thus far, and continue to persuade individual senators like Baker, Glenn and others to express public reservations about the pact that would tend to push them toward opposing it.

"This delay is a story of lost opportunities," one friend of SALT on Capitol Hill observed yesterday. "The administration could have been using it to win friends for the treaty. Well, maybe they're starting to be effective now."