The fate of both Jimmy Carter's beleaguered presidency and U.S.-Soviet relations depends on whether the president sticks to the tone he himself sounded in this steely, closed-door rebuke of the Soviets on Sept. 20: "They're lying."

Such straightforward talk is unusual for Carter. He was not in a word-mincing mood as he outlined his hopes and fears that day before congressional leaders and administration officials. Six days earlier in another private White House briefing, he had used a similar phrase: "They lied to us."

Such talk, however, could prove misleading as the showdown over the Soviet combat brigade approaches. Nobody knows whether he will follow national security adviser Zbigniew Brzezinski's advice and order a balanced buildup of U.S. power in response to Moscow's intransigence or whether he will maneuver out of the crisis with a face-saving festure, as the State Department would prefer.

This time, Carter cannot pick evenly between Brzezinsky and Secretary of State Cyrus Vance as he did in his bifurcated Annapolis speech in June 1978. This time he must choose.

Some of Vance's lieutenants -- and probably Vance himself -- are counseling super-caution. Vance's Soviet expert, Dr. Marshall Shulman, was not at all happy when Vance laid down even the limited stricture to the Kremlin that "the maintenance of the status quo" is unacceptable. To Shulman, even that cautious formulation restricted U.S. freedom of action to back down, in case the Soviet Union should refuse to change the status quo.

Shulman's inside counsel to Vance to risk nothing with Moscow that might upset the SALT II timetable was the exact opposite of the mood in the Cabinet Room when Carter and the congressional leaders assembled Thursday morning. Carter described what one White House aide called the "unrelievably tough attitude of the Soviets" toward the U.S. demand. Sen. Jacob Javits, the Senate's single strongest Republican supporter of SALT II, responded with a surprising appeal to postpone Senate consideration of the treaty.

Carter flatly rejected Javits' proposal to shelve it until the Russians remove their combat troops from Cuba. Nevertheless, there are clues pointing to a hardening presidential attitude.

Carter was furious when Undersecretary of State David Newsom, in a background briefing for newsmen, suggested the Soviet brigade might be in Cuba "to demonstrate large-unit tactics and field maneuvers to Cuban soldiers," not for combat. That argument made it seem the administration was looking for a way out.

"Jimmy Carter is a patient man who is losing his patience," one of Carter's senior aides, not associated with foreigh policy, told us. That implies -- but does not guarantee -- that the president will line up with Brzezinski if the Soviets continue to stonewall. The aide guesses this will be Carter's course.

Brzezinski has presented Carter and the National Security Council with options to force a Soviet retreat: a buildup of U.S. naval forces in the Caribbean; breaking off U.S.-Soviet talks on demilitarizing the Indian Ocean and on force reductions in Central Europe; highly visible offers of the sale of Western arms to Communist China. Eventually, if there is no Soviet backdown, Washington could clamp down on U.S. exports of high technology desperately wanted by Moscow.

That is only the start of what Brzezinski wants. Spotlighting the contradictory advice that Carter has received from Vance and Brzezinski ever since he became president, Brzezinski is making a strong case for Carter to go beyond the Soviet combat brigade in his answer to Moscow and demand new restraints on Cuba's worldwide subversive role.

The Brzezinski NSC staff was angry with the State Department's Sept. 15 answer to questions about why another OSA-class vessel was being towed from the Soviet Union to Cuba (it is now traversing the Canary Islands). Not to worry, said the department; a "routine" transfer of military equipment. To the Brzezinski apparatus, that official State Department reply undermined the formal U.S. protest to the Kremlin last December over the dispatch to Cuba of Soviet arms, including MiG23 aircraft.

Contradictory advice has marked every step of Jimmy Carter's foreign-policy course, with the president more often than not siding with Vance's State Department. The importance of how he reacts in the days ahead to the expected Soviet refusal to remove its combat brigade from Cuba dwarfs past choices.

Boldness could redeem the United States, however belatedly, in the struggle with Soviet power; too much caution will confirm the U.S. failure of will. Almost incidentally, Carter's slender chances for a second term will hang in the balance.