On the day after his speech to the nation on Soviet troops in Cuba, President Carter appeared to have lost some ground among the swing senators who will determine the fate of his SALT accord. One of them called Carter "a gullible pacifist."

Though White House officials took heart from some early senatorial comment on Monday night's speech and welcomed the prospect that the full Senate might soon debate the strategic arms limitation treaty, they also said the speech could not be expected to ignite any new enthusiasm for SALT II.

Senate supporters of the treaty said they thought Carter had revived the debate on the accord on its own merits. But some uncommitted senators were critical.

Sen. Henry L. Bellmon (R-Okla.), a respected moderate, all but declared outright opposition to SALT II yesterday after accusing Carter of adopting "the position of a gullible pacifist in his speech."

Sen. S. I. Hayakawa (R-Calif.), whom administration lobbyists have counted as a potential SALT supporter, accused Carter of accepting the Cuban status quo the president previously had called unacceptable and recommended that "all discussion of SALT II should be suspended until the Soviet Union has withdrawn all combat troops from Cuba."

Minority Leader Howard H. Baker Jr. (Tenn.) also lashed out at Carter yesterday, accusing the president of doing "nothing at all" to alter the situation in Cuba.

The White House got one crucial bit of good news from Sen. Frank Church (D-Idaho), who reiterated more specifically yesterday his willingness to let the Foreign Relations Committee begin marking up SALT II later this month.

But Church also predicted more explicitly than previously that SALT would be approved only with some sort of condition or reservation requiring Carter to certify that Soviet combat troops were no longer in Cuba before the treaty could take effect.

Sen. Sam Nunn (D-Ga.), to whom a number of uncommitted senators now look for a lead on SALT II, said firmly yesterday that he would vote "no" on the treaty if it is brought up for final approval before he has had a chance to study the administration's fiscal 1981 defense budget and its revised five-year defense plan.

Normally those documents would not be published until January. Nunn said yesterday the administration could produce them sooner if it wanted to. He added that the documents would have to show a determined U.S. effort to increase military preparedness in the 1980s.

Nunn said the issue of the Soviet brigade in Cuba "got blown out of all proportion," but added that he saw a serious problem for the United States in the evolving Soviet-Cuban military alliance and the Soviet conventional buildup on Cuba.

Carter administration officials yesterday insisted that the president's speech had at least ended the steady erosion of support for SALT that seemed to be set off by the revelation of Soviet combat troops in Cuba. They expressed optimism that a Senate floor debate on the treaty would produce 67 votes for it, the number needed for approval.

Sen. Gary Hart (D-Colo.), a strong SALT supporter, said the president's speech had restored "80 percent, maybe 90 percent" of the support SALT enjoyed before the revelation of the troops. By Hart's own count, however, the treaty still would be in trouble, because it never had any extra support in the Senate.

Another SALT supporter, Sen. Alan Cranston (D-Calif.) said it was time "to put aside childish things and childish ways, and it is time to recognize that we are a grown-up, mature nation."

"Let us stop trembling and shaking at everything the Soviet Union does that we don't like," Cranston said on the Senate floor.

At the opposite extreme, Sen. Jesse Helms (R-N.C.) declared that "the failure to change the status quo in Cuba is tantamount to an admission that the Soviets now hold strategic superiority so great that the United States cannot resist the Soviet will."

Majority Leader Robert C. Byrd (W. Va.) quickly responded that his colleague seemed to be proposing a nuclear confrontation. "Talk is cheap," Byrd said, praising Carter for placing the Cuba flap "in proper perspective" in his speech.

In the halls of the Capitol yesterday, the fate of SALT II again became a prime topic of conversation. But prophets on both sides of the issue seemed reluctant to make new predictions, given the unpredictable course of the debate so far.

Some of the discussion must be essentially procedural. Though the Foreign Relations Committee could report the treaty to the full Senate by the end of October, Nunn's insistence on seeing next year's defense budget before he votes on SALT could prevent a quick debate in November.

Baker yesterday said he favored a three or four-week televised floor debate as soon as possible. Byrd hinted he might be reconsidering his earlier opposition to televising the debate.

The heated rhetoric employed by many senators yesterday suggested that the Senate is still far from attaining the cool, deliberative atmosphere that SALT backers think would be most conducive to the treaty.

Baker's attack on Carter was unusually strong. The minority leader, an unannounced but probable candidate for president, said there was only one option Carter did not have on the Cuban issue -- "to do nothing at all."

"I'm afraid what he did was nothing at all," Baker added. "In this case we stood toe to toe with the Soviet Union and, unlike 1962, this time we blinked."

Baker said Carter could have threatened to withdraw SALT II if the Soviets refused to withdraw their troops from Cuba, to suspend negotiations on most-favored-nation trade status for the Soviets or to restrict sales on high technology items such as computers. Or Carter might have declared void the 1962 Soviet-American agreement on Cuba on the grounds the Soviets had violated it, Baker said.

Sen. Edward M. Kennedy (D-Mass.), a likely challenger to President Carter for the presidential nomination next year, said he hoped Carter's speech "would put this matter [of Soviet troops in Cuba] to rest and permit the Senate to go forward with the important task of ratifying the SALT agreement.