President Carter yesterday offered a slightly refined position on defense spending in 1981 and 1982 in hopes of winning over more moderate senators on the SALT treaty. But two key senators made statements raising new doubts about favorable action on the arms limitation treaty.

The two senators were Henry Bellmon (R-Okla.) and Sam Nunn (D-Ga.). The widely respected Bellmon urged his colleagues to delay action on SALT II and to appoint a special select committee to evaluate the country's foreign policy and defense needs.

"So far as I am concerned right now," Bellmon said, "SALT II, considered only on its technical merits as an arms control agreement, is unacceptable."

The White House had been hoping for Bellmon's support for SALT II, and by some headcounts his defection could be crucial.

Nunn, whose support for the treaty apparently is also crucial if it is to win two-thirds Senate approval, said yesterday, "It could well be beneficial to the country" if SALT II were defeated because the Senate decided that the "overall military trend between the United States and the Soviet Union" was unacceptably unfavorable to the United States.

This was the first time Nunn had publicly suggested there might be a clear benefit to the United States in rejecting SALT II. Nunn made his remarks in a luncheon meeting with reporters.

However, Nunn also said it would be a mistake to reject SALT II for narrower reasons such as its provisions permitting the Soviets to have heavier payloads on their rockets than the United States can have.

Previously Nunn has said that SALT II would be acceptable provided the United States undertakes an aggressive policy to improve military forces in response to continuing Soviet force modernization and expansion. He is one of the senators who has been pressing the White House to commit more money for defense.

President Carter addressed that issue yesterday in a letter to Sen. Ernest F. Hollings (D-S.C.), whose amendment to the annual budget resolution authorizing large increases in defense spending is scheduled to come up in the Senate today.

Hollings, Nunn and several other senators met Carter last week to press for new commitments to a 3 percent increase in defense spending this year, after inflation, and 5 percent increases in each of the next two years.

The Carter administration has accepted the 3 percent increase this year, which would add about $3.2 billion to the fiscal 1980 budget, bringing defense outlays for the year to $130.6 billion. But the president has refused to go along with the proposal to add 5 percent to both the 1981 and 1982 defense budgets after inflation.

He did offer a slight concession in his letter to Hollings yesterday, saying "if in my judgment more than 3 percent is required" to meet defense needs for 1981 and 1982, "I can assure you that I will request it."

Hollings said last night he was pleased to have the president's support for the 3 percent increases, but "sad" that he would not support 5 percent. Senate sources and administration head counters agreed yesterday that Hollings has a chance of winning a Senate vote on the 5 percent proposals today.

Both the administration and senators pressing for more defense spending agree that this issue is essentially symbolic for now, but both are taking the symbolism seriously.

In his statement yesterday Bellmon -- ranking Republican on the Senate Budget Committee -- said he thought the current fuss over defense spending disguised the fact that the Senate does not know what the country's defense needs are. "Now is a time for a temperate, well-reasoned, factual, thoughtful review of where we are and where we are going in U.S. foreign and defense policy," Bellmon said, calling for a new select committee inquiry.

"In my opinion," Bellmon said, "under present conditions, the environment in which the Senate must consider SALT is intolerable and it would be foolhardy to rush toward a vote." He proposed that the select committee report back to the Senate by March 1.

Bellmon's proposal could reach the Senate floor for a vote, and some Senate sources speculated it might just suit the current mood of the Senate if it comes up soon.

If Hollings' 5 percent proposal is defeated by the Senate today, or perhaps tomorrow, Sen. Gary Hart (D-Colo.) will introduce a compromise proposal calling for 3 percent increases in each of the next three years, the formula preferred by Cater.

The administration hopes this proposal will be approved, and that it will be enough eventually to satisfy senators concerned about defense without alienating more dovish members, thus preserving a coalition that could eventually put over the SALT treaty. This formula may not work, however, administration officials concede.