Despite the Soviet invasion of Afghanistan and the uncertain future of U.S.-Soviet relations, the Carter administration seems ready to confront Republican presidential challenger Ronald Reagan on the question of moving ahead with arms control agreements with Moscow.

Zbigniew Brzezinski, the president's national security affairs adviser, says the arms control issue will be "perhaps the key question" in the election. s

Brzezinski believes that as long as the United States is "firm" in containing Soviet expansionism, this country can safely move ahead with talks to control atomic arms with Moscow even though Soviet troops remain in Afghanistan.

He argues that limitations on the nuclear-tipped missile arsenals of both superpowers remain in the U.S. national interest, despite Moscow's invasion, and that those who reject this notion are toying with a dangerous new arms race with an uncertain outcome.

Brzezinski outlined his views in a recent discussion with this reporter and later agreed to make some of his remarks public.

The platform approved at last week's Republican convention rejects, as "fatally flawed," the strategic arms limitation treaty (SALT II) with Moscow, which has been signed by Carter and Soviet chief Leonid I. Brezhnev but has not been approved by the Senate.

Reagan has argued that the United States must first challenge the Soviets to an accelerated arms race -- which he believes the United States will win -- before Moscow will agree to real arms control.

The GOP platform calls for overall U.S. military superiority against the Soviets. But Brzezinski counters that "those who reject arms control arrangements, and only talk about going back to nuclear superiority, are doing two things which are undersible."

"One, they are harking back to an age which cannot be retrieved, namely the situation in the 1950s" when the United States emerged from World War II with overwhelming atomic superiority over Moscow.

"Secondly," he says, "they are fueling the arms race, which is destabilizing and, in the short run, perhaps even advantageous to the Soviets because they have the momentum," meaning Moscow has many more missile-producing lines operating than does the United States.

SALT II essentially puts equal limits on the number of missiles on both sides.

At a breakfast meeting with reporters yesterday, Secretary of State Edmund S. Muskie said he thought SALT was "indispensable" to U.S. security and that the Soviets would never accept U.S. superiority, and thus such talk was certain to trigger a continued arms race of "enormous cost that can't be won."

"My view throughout," Brzezinski says, "has been that if we are firm in containing Soviet expansionism, we can at the same time equally and actively, promote detente, including arms control arrangements.

"We believe certain aspects of the [U.S.-Soviet] relationship such as arms control can and should be developed irrespective of other issues."

Muskie told reporters last month that he believed the Afghan situation and arms control were "separable" and that the conflict in Afghanistan had "even elevated the question of arms control as an important security issue" in his mind.

Thus, the two key supporting figures in the administration appear to be ready to defend continued arms control talks with Moscow, even though they say political conditions here are highly unlikely to allow any movement in those talks -- certainly in the main negotiation, the stalled SALT II talks -- before the November election.

Brzezinski believes the U.S. public is prepared to support this dual policy of arms control and firmness toward Moscow, but only if it is convinced that the policy of firmness "is genuine and effective."

He believes Carter's response to the Soviet invasion -- including the Olympic boycott, grain and technology embargoes and a regional defense buildup -- has been firm and that the president has credibility on this point.

"Because of that," Brzezinski says, "I think we are in a good position to argue that we should go ahead with those arms control arrangements which are feasible."

Aside from SALT II, the United States has taken the first small steps toward responding to recent indications from Brezhnev that Moscow may be willing to negotiate limits on medium-range nuclear missiles based in Europe.

Though nothing is apt to happen before the November election in Either negotiations, the reasons are different.

In the case of the Euromissiles, Brzezinski says there is no political impediment to moving ahead quickly. Rather, it is a practical matter because the issue is extremely complex and will take prolonged study within the administration and among U.S. allies before the western position on such talks can be hammered out.

"It certainly will not be easy to be ready before the elections to have initial explorations [with the Soviets], but it's not inconceivable that enough sorting out will be done so that some preliminary contact will take place," Brzezinski says.

For SALT, however, the question is whether it can be approved by the Senate and "that, as a practical matter, is not likely before the elections simply because the congressional calendar doesn't permit it," he says.

Furthermore, he adds, "the Republicans right now are committed by their platform to be against it, and I think in any vote before the elections they'll have to be very heavily affected by that. After the elections, there is going to be more of an inclination to look at SALT on its own merits."

In Muskie's view, it will be "a very steep, uphill climb" politically to get SALT ratified.

Unstated is the fact that many Democrats who favor SALT are also not anxious to see it come to vote before the election, sensing a possible political liability in being perceived as advocating doing any sort of business with Moscow at this time.

While believing that arms control with Moscow can, and should, go forward, Brzezinski takes a dark view of the global situation if the Soviets do not withdraw their troops from Afghanistan.

"Between 1945 and 1955," Brzezinski argues, "we stopped Soviet Expansionism westward and eastward and we created a system of deterrence which then made detente possible.

"For reasons perhaps connected with Soviet power, perhaps connected with a crisis in America, the Soviets are now pushing southward through Afghanistan at natural resources vital to the survival of Western Europe and the Far East, and through Cambodia and Thailand potentially to vital sea lanes.

"If we can contain this push and deter it, we can resurrect detente. If we do not, we're likely to have a very unstable situation in the 1980s, one in which our vital interests would be generally jeopardized.

"Afghanistan, in a sense, is a litmus test of whether American-Soviet relations can go quickly back to normal through a constructive and decent resolution of the issue, or whether Afghanistan is the symptom of the deeper manifestation -- Soviet expanisionism -- which will take many more years to work itself out."

But the Soviets show every sign of digging in for the long haul in Afghanistan, and disengaging from a battlefield is a hard chore for any country.

Brzezinski agrees and estimates the changes as probably less than 50-50 that the Soviets will reconsider. Still, he believes that international condem-nation of Soviet policy has been so widespread and the costs so high that there is at least a chance that Soviet leadership might take a new course.

But is the United States capable of sustaining a long-term defense buildup over the Afghanistan situation -- especially if the issue fades from the front pages and if the U.S. hostages are returned from Iran, another factor that could lower the national temperature here?

On the one hand, Brzezinski acknowledges that public opinion often shifts quickly.

"But I think the public today senses that our strategic position is being threatened, that there has to be a sustained response . . . to a challenge that itself is highly sustained and extensive in scope," he says.

"I don't believe that trend is going to be totally reversed, even if there is some sudden improvement in the immediate situation. I think the country, the public, doesn't want the United States to be in an inferior position and is prepared to make the necessary sacrifice, provided these sacrifices are clearly stated and the reasons for them articulated."