Pakistan is treating the United States with disdain. In its pursuit of a nuclear weapons capability, Pakistan is breaking its commitments to the United States and is flouting U.S. laws. Pakistan's conduct casts doubt on the viability of its current security assistance relationship with the United States.

Earlier this month a Pakistani agent was arrested in Philadelphia for attempting to export a specialty steel for use in Pakistan's nuclear weapons program. This case follows a much publicized 1984 incident involving the attempted export of electronic switches designed to detonate nuclear explosives. The furor set off by the 1984 case led to Pakistani promises not to acquire nuclear technology in the United States and to a new law barring U.S. assistance to nations that attempt to acquire illegally nuclear technology in this country. It is this commitment and this law that Pakistan has now so blatantly violated.

Pakistan has treated other promises to the United States with a comparably cavalier attitude. It continues to act in a manner inconsistent with its promise not to manufacture nuclear weapons, and it has violated a pledge to President Reagan not to enrich uranium above 5 percent. In the area of narcotics control, Pakistan promised in 1985 a major effort to reduce opium production. In 1986 such production increased.

With regard to terrorism, Pakistan promised an uncompromising stance. Specifically, we were told the four Palestinians apprehended for last year's murderous hijacking of a Pan Am 747 would meet "Pakistani justice," a term widely understood to mean a prompt rendezvous with Allah. Nearly a year later no trial has been set.

Why does the United States tolerate such a total disregard for commitments made to it, commitments that affect the vital interests of our country? The answer, in a word, is Afghanistan.

Although Pakistani restraint on its nuclear program was an explicit quid pro quo for renewed U.S. assistance, the Reagan administration has been unwilling to use the assistance program as a lever to force Pakistan to honor its nonproliferation commitments. Pakistan may respond to any such effort, the administration argues, by shutting down the aid pipeline to the Afghan Freedom Fighters. So far, Congress has bought this argument. It is, however, an argument of dubious validity.

Pakistan permits assistance to the Afghan Freedom Fighters for its own strategic and political reasons. The continued Soviet occupation of Afghanistan is far more of a threat to Pakistan than to U.S. interests. If the Soviets were to consolidate their position in Afghanistan, Pakistan would face another neighbor militarily more powerful and with oft-stated designs on Pakistani territory. Pakistan has an obvious interest in preventing such a consolidation, which is why it had permitted assistance to the Freedom Fighters for nearly two years before U.S. foreign assistance to Pakistan was resumed. A cutoff of such U.S. aid would not change Pakistan's strategic interest in maintaining the Afghan pipeline.

For domestic reasons Pakistan cannot afford to shut down the Afghan pipeline. Since the 1979 Soviet invasion Pakistan has generously played host to some 3 million Afghan refugees. These refugees, who are heavily armed, have settled mostly in Pakistan's Northwest Frontier province. It is an area long prone to separatist sentiment and whose 10 million indigenous inhabitants are tribal kinsmen of the Afghan refugees. Pakistani authorities know that a decision to retaliate against a U.S. aid cutoff by shutting down the Afghan pipeline could result in civil war.

Finally, Pakistan has diplomatic reasons to maintain the Afghan pipeline. Pakistan's closest military partner is China, a country reportedly much involved in assisting the Afghan Freedom Fighters. Another military and financial partner is Saudi Arabia, which also has a strong interest in assisting the devout Afghans fight the atheistic Soviet invaders. Retaliation would put Pakistan in the position of cutting off its Chinese-Saudi nose to spite its American face.

Pakistan has repeatedly stated that its Afghan policy is undertaken for its own reasons and is not dependent on U.S. assistance. In this case, there is every reason to believe Pakistan.

The administration has played, on Pakistan's behalf, an Afghan card that Pakistan does not possess and indeed does not claim. The result has been an American posture of weakness on matters of vital national interest.

Our failure to hold Pakistan to its nonproliferation commitment risks a disastrous nuclear arms race in South Asia and undermines the credibility of our global nonproliferation policy. The costs are also high in the areas of terrorism and narcotics.

In addition, our willingness to accommodate Pakistani deceit on the nuclear issue may be harmful to the Afghan program itself. Journalists and South Asia experts have estimated that a large part of the assistance intended for the Afghan Freedom Fighters is being diverted into Pakistani pockets. Such diversions obviously are harmful to the Afghan cause. They are made possible, in part, by a Pakistani belief that the United States is unwilling or unable to hold it accountable.

It is time to get tough with Pakistan. The policy of excuses and quiet diplomacy has failed to arrest Pakistan's nuclear program and has invited a cavalier Pakistani approach to all its commitments to the United States. If Pakistan perceives that the United States means business, it is more likely to keep its word. It will certainly respect us more.

The writer, a Democratic senator from Rhode Island, is chairman of the Foreign Relations Committee.