The Carter administration took heart yesterday from the latest Senate maneuverings on SALT II, but critics of the arms limitation treaty also expressed renewed confidence that they could block its ratification or alter the treaty substantially.

The Iranian crisis and a busy year-end legislative schedule have diverted attention from SALT II for weeks, but the subject came back to the fore yesterday after publication of a letter from 19 senators to President Carter asking for further treaty concessions and clarifications.

Today the Senate Armed Services Committee is to take up a proposed committee report sharply critical of SALT II, and sources on both sides of the issue predicted last night that the report would be approved. The administration hopes to minimize the impact of this attack on the treaty.

Backers of the treaty said they were encouraged that the 19 senators who wrote to Carter had publicly declared themselves still undecided, and showed a willingness to continue negotiating with the administration. The list of 19 included four or five senators previously considered likely opponents of SALT II, and an equal number who have remained undecided throughout the preliminary debate on the treaty. The other signers had been counted as likely supporters of SALT II approval.

However, conversations with a number of the senators who signed the letter, who were inclined against SALT before they signed remain so now. "I lean against the SALT treaty and I have not changed that," said Sen. Pete V. Domenici (R-N.M.) in a typical comment.

Domenici, Alan K. Simpson (R-Wyo.) and Dennis DeConcini (D-Ariz.), all indicated that they approved the letter -- drafted primarily by Sam Nunn (D-Ga.) -- because it expressed concerns they shared, and not because they were any closer to approving the treaty. All are learning against.

The fate of SALT II now appears to depend on several variables, some inside the Senate and some way outside the Capitol. Among them:

Timing. Senate consideration of the treaty has been postposted until next year, and the White House and Senate leaders agree that debates should not begin while the Iranian crisis continues. Senate Majority Whip Alan Cranston (D-Calif.), predicted yesterday that the treaty could come to the floor in late January or early February, and be voted on by early April, but this depends on events in Iran as well as the political mood in the Senate.

Many senators, including some of the 19 who signed the Nunn letter, seem to prefer delaying a vote until after the 1980 elections. The letter said it would be better to delay action than to defeat the treaty. However, Nunn said yesterday, "We're not . . . advocating delay at this time."

The defense spending issue, Nunn has made his SALT vote dependent on pledges from the admistration to increase defense spending, and the White House has responded by promising to raise the Pentagon's budget billions of dollars above previous estimates to try to satisfy him. Nunn has reacted positively, but still reserves his position on SALT, and even raised his ante somewhat in the letter by asking for possible new stategic weapons programs on top of the conventional programs he demanded earlier.

Several senators have adopted Nunn's position, at least in general terms. So has former secretary of state Henry A. Kissinger, who is thought to have some potential influence over undecided Republican senators who probably would cost the deciding votes if the treaty comes to a final vote.

Nunn, who has emerged as a focal point for SALT politics and thus a center of public attention, has generally enjoyed high esteem among his colleagues, particularly on defense matters. Yesterday, however, several senators and Senate aides who have been friendly to him said they thought he was now pressing his case too hard and seeking too much public attention. "Sam got what he wanted," one said in a reference to the defense budget, "and he ought to lay off."

The SALT II issue. A number of senators have decided that SALT II should contain some form of senatorial insistence that SALT III include substantial reductions in the two superpowers' strategic weapons. The White House has expressed willingness to certain ideas of this kind.

The crucial issue here is whether to demand a formal amendment to SALT II stipulating that the treaty would lapse in the absence of agreed arms reductions by a fixed date. Senate leaders and the White House hope to avoid an amendment that would require Soviet approval.

But others demand the formal amendment. Sen. Daniel Patrick Moynihan (D-N.Y.) said yesterday that "it has not sunk in" to the Carter administration that the Senate is fed up with a SALT process that so far has resulted in arms escalation, not reduction.

"They will take us seriously in time," Moynihan predicted.