In a tense day of meetings and strategy sessions, the Carter White House moved yesterday to protect its endangered SALT II pact against damaging political winds and tides that have arisen and intensified as ominously as a September hurricane.

The day began with an emotional, sometimes angry, meeting between President Carter and his principal associates and a group of senators who are pressing the White House to increase defense spending. It ended with a surprise early evening meeting between one of those senators, Sam Nunn (D-Ga.), and President Carter that lasted for more than an hour.

Neither Nunn or the White House would discuss the second Nunn meeting, which was prompted by a personal invitation during the day from President Carter. But a senior official said their conversation may have been the single most important exchange in the SALT debate thus far.

There was obvious anxiety in the highest reaches of the administration yesterday that the SALT treaty might be slipping down a political drain that has been greased by revelations of Soviet combat troops in Cuba. Last night White House officials expressed hope that the worst had passed, at least temporarily.

"We've got to solve the Cuban problem to make progress," one senior official observed last night.

Opponents of SALT in the Senate, meanwhile, exulted yesterday that the political momentum seemed to be flowing their way. The announced opposition of Sen. Russell B. Long (D-La.), the flap about Soviet troops in Cuba and the continuing debate over the best level for defense spending all have tended to weaken the pro-treaty arguments, many of the SALT critics believe.

Key members of the Carter administration acknowledge privately that they have lost the momentum. "We desperately need to get back on the offensive," one said.

In the short run, the administration is anxious to avoid any more declarations like Long's that might suggest a wholesale abandonment of SALT II by moderate and conservative senators. One reason for Carter's second invitation to Nunn last night, apparently, was to try to convince the Georgian -- now clearly the most immpotant single senator in the SALT debate -- to be patient and to hold off on any definitive statements about his position on the treaty.

Nunn is a widely respected expert on military affairs to whom at least half a dozen senators are looking for a lead in their own thinking about how to vote on SALT II.

This limited objective may have been achieved. Leaving the White House under dark, threatening clouds last night, Nunn told a reporter that he will wait to see the administration's new five-year defense budget before making up his mind finally on SALT II.

Nunn predicted that the SALT debate now is so mired that a final vote cannot come before late November or December, giving the administration plenty of time to produce such a longterm budget. "I'm tying my vote on SALT to what we do in defense [spending over the next few years] and to presidential leadership on national defense," Nunn said.

He noted that he and President Carter agreed that a 3 percent increase in real spending is the most that can be absorbed sensibly in the defense budget this year. But he reiterated that a 5 percent real increase will be necessary in the 1981 and 1982 budgets to satisfy the Pentagon's plans for needed improvements in American forces.

Nunn's insistence on the 5 percent figure in the first White House meeting yesterday was stronger than administration officials had expected. Nunn's firm position was once factor that led the president to invite him for a second visit to the White House, informed sources said.

Others in the morning meeting also pressed Carter forcefully. Sen. Henry M. Jackson (D-Wash.) accused the administration of foot-dragging on strategic weapons programs, a charge that angered Defense Secretary Harold Brown, who was in the meeting.

Sen. Ernest F. (Fritz) Hollings (D-S.C.) told Carter that the commander in chief "has to lead," Hollings later told a reporter. "It was a standoff," Hollings said of the meeting, reporting that Carter did not accept the arguments for further increases in defense spending. "He was looking anxious," Hollings said of Carter."He's beleaguered. I felt a little sorry for him."

Meanwhile, the State Department attempted to check a wave of speculation arising from the disclosure Wednesday that the Soviet troops in Cuba may be there to demonstrate large unit maneuvers to Cuban soldiers.

Spokesman Hodding Carter declared this is only one of "a whole series" of possible missions for the Soviet brigade under study by U.S. intelligence and he denied that the disclosure was intended to be "a trial balloon or in any way a signal" of a possible way out of the current U.S.-Soviet dispute.

Secretary of State Cyrus R. Vance is scheduled to meet Soviet Ambassador Anatoliy F. Dobrynin again this morning, in their third meeting to discuss Soviet troops in Cuba.

Staff writer Don Obendorfer contributed to this article.