The most ardent -- and until recently most optimistic -- Senate supporters of President Carter's SALT II treaty see virtually no way now to revive its sagging prospects.

Both propponents and critics agree that the strategic arms limitation treaty with Moscow can be saved only by some turn of events as unexpected as the one that put the pact in its present difficulty -- the discovery of Soviet combat troops in Cuba.

Even without optimism, though, the treaty's supporters retain hope -- hope that the Cuba flap will somehow go away, that President Carter can somehow regain the initiative, or that Senate elders like Robert C. Byrd (D-W. Va.) and John C. Stennis (D-Miss.) can still save the day.

The only SALT consensus discernible today is that the treaty is in political trouble. When the first month of Senate hearings ended in early August, there were signs of an emerging agreement among a strong majority that SALT was a small but useful step, provided it was accompanied by rigorous defense programs and a firm foreign policy. If that notion now collapses along with the treaty, there is no evidence of a successor consensus that could take its place.

One Senate aide said this week that defeating or simply shelving the SALT treaty until after the 1980 elections could leave the United States without a broadly supported national security policy, and without a president capable of formulating or executing a national security policy. That is now the prospect.

Hard-line critics of SALT II and the country's defense posture in general are not upset at this prospect, of course. Many of them predict that Congress and the country will soon have to accept their view that the United States needs a much bigger, much more expensive defense program.

How this new situation developed may be of interest to historians for many years, but several important explanations can already by identified.

The Cuban flap is certainly the starting point. It is difficult to find anyone in or around the Senate who thinks the Carter administration reacted well to the discovery by its own intelligence agencies that a Soviet combat "brigade" has been operating for some unknown number of years in Cuba.

This discovery was provoked by the White House itself, which asked the intelligence community to conduct an intense investigation of the Soviet presence in Cuba. The administration had months to prepare for the bad news that this intense review might produce, but apparently no preparations were made. Instead, when the analysts decided that there really were Soviet combat troops in Cuba, the White House had no plan for dealing with this discovery.

Eventually the third-ranking official in the State Department, David Newsome, telephoned several members of Congress to tell them the news. One he called was Frank Church (D-Idaho), the chairman of the Senate Foreign Relations Committee, who had argued for months that the SALT treaty should not be "linked" to other issues in Soviet-American relations.

Church took it upon himself to announce the intelligence community's new findings, arguing later that he was trying to save the administration from its own folly by at least letting the announcement come from a "responsible" quarter. Many senators privately express wonderment that President Carter did not seize the initiative by personally making the initial announcement.

Instead, said one, Carter waited for days, then "announced that the status quo was unacceptable before he knew what the status quo was."

For his part, Church suddenly became an advocate of linkage, declaring that the Soviets should withdraw their combat troops from Cuba, declaring that if they didn't, there was no prospect that the Senate would approve SALT II. With a pro-SALT liberal like Church in that posture, senators who wanted to establish a public position to the right of his could only make stronger pro-linkage statements, as many have.

As these events were unfolding, the administration was discovering that the hopes it had placed in Sen. Sam Nunn (D-Ga.) to emerge as a principal advocate of the treaty would not be easily realized.

In late July Nunn announced that he could support the treaty, but only if it were accompanied by a significant increase in the defense budget -- perhaps 5 percent a year for five years or more. The treaty itself, he added, looked all right to him.

The administration seized on his comments as the long-awaited indication that Nunn would support SALT. But the administration apparently felt that Nunn's conditions could be satisfied without any basic alteration in its own rhetoric or behavior. Nunn wanted more than that.

As weeks went by, and the press continued to report that the administration didn't think there was any sensible way to spend more than a 3 percent annual increase in the defense budget, Nunn got angry. He apparently came within a hair of announcing his probable opposition to SALT earlier this month, and was only dissuaded by a passionate personal plea from President Carter. Nevertheless, he is now described by intimates as profoundly distrustful of the administration's true intentions, and not at all likely to step forward as a defender of the SALT treaty.

Nunn enjoys a special status as a moderate southern expert on defense issues; many colleagues look to him to take a lead that they can follow. Without his support, the treaty's chances are negligible.

Another development that has contributed -- though less tangibly -- to the current situation is the emergence of Sen. Edward M. Kennedy (D-Mass.) as a likely challenger to Carter's reelection. The initial impact of Kennedy's move toward a declared candidacy has been to emphasize the president's own political weakness. From the beginning, the SALT treaty has been a hostage to Carter's standing at the time the Senate finally votes.

For nearly two years, senators interested in SALT have been making the same point: faith in the commander-in-chief was bound to influence many senators' votes. Today faith in the commander-in-chief is virtually impossible to find on Capitol Hill. Senators who don't fault his strategies fault his tactics; almost no one rises publicly to defend Carter.

Today the strongest instinct in the Senate is to put off SALT vote indefinitely, which effectively means until 1981, after the presidential election.

One important figure seems likely to challenge that mood, however. Robert Byrd, the majority leader, though he has not announced his position, has become an ardent supporter of SALT. In recent days he has received a stream of visitors, showing many of them his personally annotated copies of the July SALT hearings, telling them how important the treaty is, and declaring that its fate should not be tied to the Cuban issue or to Jimmy Carter.

Byrd seems to believe that the Senate -- his senate -- must face up to its constitutional duty to consider the treaty. Byrd will reportedly fight any attempt to shelve the treaty, or to dispose of it with some sort of sense-of-the Senate resolution that would fall short of either approval or outright rejection.

The White House knows about Byrd's attitude. The president's closest associates perceive an urgent need to reassert some direction and control in the SALT debate. Senior officials met in the White House last night to discuss how this might be done.

Supporters of SALT II in the Senate and senior administration officials insist nothing has changed the basic logic that in their view has always argued for the treaty.

Unless it is approved, they say, relations with Western Europe will be severely strained, defense policy as home will lack direction, and the pledge of non-proliferation will be forfeited, encouraging other nations to join the nuclear club.

For many months the White House has believed that the Senate would eventually have to face these and other consequences of rejecting SALT II, and therefore would finally approve it.

Opponents of the treaty have never agreed that the consequences of rejection would be so ominous. They profess willingness, sometimes eagerness, to live with whatever consequences follow rejection rather than accept a treaty they find unacceptable.

The hard-line opponent have argued for months that the United States has to be tougher in its dealings with the Soviet Union. This is an urge that has wide appeal in the Senate, and the Cuban flap seems to have exacerbated it.

"We seem to have this low-grade chauvinistic fever that we haven't been able to shake out of our system since Vietnam," a liberal senator said this week. "We're just spoiling for a fight, trying to regain our lost manhood."

Is SALT II now doomed?"Don't say that," a leading opponent said yesterday with a nervous laugh. "It can come back." But this same Senate aide admitted he couldn't see how it might come back.

Said a senior Carter adviser, gamely: "Just say we had a good first half and this is only the third quarter."