With a stroke that may have redrawn the battle lines in the SALT II debate, Sen. Sam Nunn (D-Ga.) said yesterday that the Carter administration can gain his vote for the treaty by committing itself to substantially increased defense spending.

Nunn said he had seen a good case for approving the pact but only if the is accompanied by additions to the next defense budget of about $7 billion on top of increase already planned by the administration.

By quitting the school of senators who criticize SALT II on its merits, Nunn gave a substantial boost to the treaty's prospects. He also established himself as principal spokesman for the large number of moderate senators who seem increasingly willing to accept SALT II provided it is accompanied by billions of dollars in new defense outlays.

Last night, senior administration officials met to discuss how they could satisfy Nunn's concerns. Informed sources indicated that the administration is ready to negotiate with the Georgian.

The White House has long regarded Nunn's SALT vote as potentially critical, since he has an impeccable record as a proponent of a strong national defense and a reputation for good political sense. Numerous senators, including a large bloc of southern moderates, are looking to Nunn for Guidance in the SALT debate.

Administration officials expressed satisfaction yesterday with Nunn's statement, which the senator delivered at the Armed Services Committee's SALT II hearing. These officials emphasized Nunn's implicit support for the treaty, which he made explicit to a reporter later, repeating the proviso that the treaty must be accompanied by accelerated defense spending.

But Nunn's conditions are expensive and could cost the Carter administration painful billions of dollars in its 1981 preelection budget unless Nunn moves off the firm position he adopted yesterday. In his statement, he called for real increases, after in inflation, of 4 to 5 percent in the proposed $122.7 billion fiscal 1980 defense budget -- annually for the indefinite future.

Despite the impact on efforts to balance the federal budget on the eve of a recession, Nunn's proposals seem to suit an emerging mood at least on the two Senate committees, Armed Services and Foreign Relations, which have been considering SALT II.

"What's come out of these hearings", a senior administration official said last night, "is a much more serious concern about where we stand vis-a-vis the Russians." The White House is said to be ready to deal with this concern by pledging stronger defense efforts, though there is no eagerness to go to the numbers Nunn proposed yesterday.

One administration official said the senators seem most concerned about Carter's commitment to a strong defense, not Congress's attitude toward spending the money. That concern will have to be satisfied, this official said, "though we may have to break a few bones around here to do it."

Nunn specifically declined to offer a shopping list of defense programs that he would require in return for his SALT vote. (He did offer such a list in a speech April 30, without tying it directly to SALT II.)

Instead, Nunn said, he wanted a new "commitment from the president to the people" that the United States will go well beyond current plans for defense spending in the early 1980s.

"Only presidential leadership can change these trends," Nunn said, referring to the steady growth of Soviet military power compared to U.S. power.

Sen. Gary Hart (D-Colo.), a pro-SALT member of Armed Services committee, replied in a statement that the treaty shoughld not be a blamed for "deep divisions in our society" that have held down defense expenditures in recent years.

Sen. John C. Culver (D-Lowa), another Salt supporter, said the "same voices" who have insisted on a balanced federal budget are now asking for military programs that could create a $60 billion budget deficit.

Nunn's statement was the high point of yesterday's SALT hearings, in which the Joint Chiefs of Staff continued to answer senators' questions.

Critics of the treaty asked the chiefs numerous questions about their earlier statements that the United States would lose "essential equivalence" with the Soviet Union in the early 1980s, hopefully regaining it by the mid-1980s. Responding carefully, the chiefs several times said they thought the Soviets would have an advantage over the United States in this period.

After the lunch break, Gen. David C. Jones, chairman of the joint chiefs, sought to redress what he evidently thought was an overly gloomy impression by emphasizing that throughout the 1980s the United States would retain the power to deter a nuclear attack or to retaliate successfully if attacked.