The Carter administration is struggling with painful calculations about the potential impact of a further delay in Senate considerations of SALT II, a prospect that now appears unavoidable.

President Carter has refused to "line" SALT II to the Soviet intervention in Afghanistan, and continues to say that the treaty was not signed as a favor to Soviets, and remains in the American national interest. But a senior White House official acknowledged yesterday that the events in Afghanistan have affected the timing of the strategic arms limitation treaty debate, and that can only mean further delay.

Senate Majority Leader Robert C. Byrd (D-W.Va.) has said nothing about the SALT debate since the coup in Afghanistan, and he has avoided press inquiries on the subject.But knowledgeable sources said yesterday that Byrd feels it is now impossible to hold a SALT debate in February, as he previously planned.

Some administration officials and some pro-SALT sources in the Senate continue to express hope that a SALT debate could occur later in the year, and that the treaty could still be approved by a two-thirds vote. But other sources dismissed such speculation as pipe-dreaming.

The problems raised by further delay for the Carter administration are numerous. Among them:

American plans for new strategic weapons programs will likely have to be altered. The administration has been assuming that its principal new strategic weapons system can be the MX mobile missile, but MX only makes sense in the context of a strategic arms competition governed by SALT II.

This is true because the Soviets could relatively easily neutralize the MX system by adding new warheads to their existing or new land-based missiles beyond the limits on warheads imposed by the treaty. Without the SALT II limits, the Soviets could move relatively easily in this direction.

"Without a SALT treaty, we will have to explore alternatives to MX," one senior official said yesterday. Such alternatives could prove both complicated and costly.

Continued delay of SALT II gives the Soviets, in effect, the option to unilaterally kill the SALT process at any time. They could do this in several ways.

One would be to cease respecting the voluntary extension of the SALT I agreements on offensive arms that theoretically lapsed at the time SALT II was signed. As recently as last month the Soviets reported on their continuing dismantling of outdated Yankee-class submarines as required by those SALT I agreements, but the Soviets could decide at any moment to stop adhering to a pact that now has no formal status. Such a Soviet decision would likely unravel the strategic situation.

Another Soviet option would be to violate some provision of the unratified SALT II. For example, they could encode the radio signals from missile tests that the treaty would bind them not to encode. Or they could test new missiles or additional warheads for existing missiles that violate SALT II limits.

Theoretically the Soviets have always had these options while the Senate was considering the treaty. But now that serious new tensions in the Soviet-American relationship seem inevitable, the chances that they might be exercised becomes much greater.

Carter administration officials express grave alarm at the prospect of a new strtegic arms competition ungoverned by SALT constraints. They have argued for months that the best argument for SALT II is the dangers that would follow its rejection, and now those dangers look palpable to these officials.

"This (crisis brought on by the intervention in Afghanistan) will open peoples' eyes a little bit" to the dangers of a world without SALT, one senior official said yesterday.

The administration has continued to plan and operate on the theory that somehow SALT II would win Senate approval in 1980. Despite the political troubles the treaty has had in the Senate, the White House retained its optimism until last week.

Still yesterday some officials argued that President Carter could save the treaty by making a forceful statement that the crisis provoked by Afghanistan actually emphasizes the need for a treaty to stabilize the most dangerous arena of Soviet-American competition.

Others inside the administration and on Capitol Hill disagreed. Several officials involved noted the extreme pressure now on senators who support SALT, particularly those running for reelection this year, and predicted that one or more of them could easily jump the SALT ship, leaving it to founder. "Our friends are really exposed," one administration official said, acknowledging the political difficulty of defending a pro-SALT position after the intervention in Afghanistan.