Once an expresssion of the sullen animosities of the Cold War, relations between the United States and Hungary are now being hailed as one of the successes of detente.

For years, the mutual suspicion was symbolized by the stubborn figure of the late Jozsef Cardinal Mindszenty, who took refuge in the U.S. Embassy in Budapest after Soviet troops crushed the 1956 Hungarian uprising. The cardinal remained in the embassy for 15 years - living reminder of the clash between totalitarinism and democracy.

Today there is an altogether different symbol for the state of U.S. Hungarian relations. St. Stephen's crown, regarded by many Hungarians as the embodiment of their country's 1,000-year-old nationhood, is on display in Budapest's National Museum after being locked away for over 20 years in the vaults fo Ft. Knox.

A cross-section of Hungarian society - young couples with their babies, old-age pensioners, schoolchildren - files steadily past the jewel-encrusted crown, which was takem to the United States after World War II rather than being allowed to fall into Communist hands. Last January it was ceremonially returned to Budapest by Secretary of State Cyrus Vance.

Soon after its return, Washington announced that it was ready to grant most-favored-nation trading status to Hungary. Thursday, the House Ways and Means Committee approved the administration proposal. Until now Romania and Poland have been the only Communist nations extended most-favored-nation status by the United States.

The blossoming friedship berween the two countries is all the more remarkable because it has coincided with a difficult period for East-West relations in general. This has been reflected in the polemics over human uncertainty over the renewal of the strategic arms limitation talks (SALTS), and U.S. concern over Soviet involvement in Africa.

The fact that U.S.-Hungarian relations can get steadily better while U.S.-Soviet relations have been getting steadily worse is partly a tribute to the political skills of the Hungarian leader and Communist Party chief, Janos Kadar. It is also a product of new Amercian policies toward Eastern Europe, aimed at encouraging the more liberal and independent governments within the Soviet bioc.

In Budapest, a bustling city on the Danube River with smart sidewalk cafes and elegantly decorated shop windows, the opening toward the West is viewed as part of a general attempt to reconcile the Hungarian people to communism. Diplomatically the most astute of all the East European leaders, Kadar - who often invokes the slogan, "He who is not against us is for us" - has devoted his career to obliterating the memory of how he

"One thing you must realize about Kadar," says a Hungarian journalist who knows him well, "is that he is a very sensitive man who never forgets anything. For the last 20 years, he has been working to be accepted by the people who once condemned him, including Western politicians."

Kadar's recipe for giving socialism a more acceptable face has been based on a gradual easing of central controls over intellectual life, travel abroad and the economy. Since the unobstrusive introduction of the so-called New Economic Mechnism in 1968, individual factories have geared their production to the demands of the market rather than to dictates of a central plan.

U.S. diplomats say that, given the constraints imposed by the Soviet Union, Hungary is doing as much as can reasonably be expected in allowing free emigration.

"Basically anyone who really wants to get out does get out - even if it occasionally takes some time. All the family reunification cases that we have raised with the government have been esttled," said an embassy official.

A big factor in Kadar's success has been his ability to persuade the Kremlin that has his liberalization program is not merely in Hungary's best interests but in the Soviet Union's as well.

"It is hardly a coincidence that the improvement in our relations with the U.S. has followed a greater Soviet acceptance of the so-called Hungarian model," said a Budapest journalist.

The new American policy toward Western Europe is attributed in Budapest to the influence of the president's national security adviser, Zbigniew Brzezinski, and his doctrine of "political polycentrism." The U.S. administration is now believed to be cultivating its relations with individual Warsaw Pact countries, rather than negotiating over their heads with Moscow.

Brzezinski, who was born in Poland and has a keen interest in Eastern European affairs, evidently believes that countries like Hungary and Poland should be treated according to their distinctive nationals characteristics and not merely as members of a monolithic Soviet-led bloc. Thus it was noticeable that Hungary virtually escaped U.S. criticism at the Belgrade conference reviewing implementations of the 1975 Helsinki declaration, while its more hard-line neighbor, Czechoslavakia, was severely attacked.

The negotiations over most-favored nation status provided an illustration of Hungary's new-found freedom of action within the bloc. For several years, the Hungarians were oliged to follow Moscow's lead in refusing to give a formal guarantee that they would allow free emigration as required by the Jackson-Vanik amendement to the U.S. Trade Act. It was argued that this amounted to an attempt to interfere in internal affairs.

Now, however, they have felt confident enough to agree to a vaguely worded compromise, undertaking to deal flexibly with emigration cases in the spirit of Helsinki.Moscow does not appear to have objected.

Hungarian officials believe that the cutting of U.S. import duties by as much as 50 percent under most-favored-nation status will lead to a big increase in trade. Hungary's prosperity depends on being able to sell abroad.

Amid all the euphoria about the improvement in relations between Washington and Budapest, there are also some ominious signs. The honey-moon has so far managed to survive the deterioration in East-West relations but few Hungarian politicians believe it can do so indefinitely.

Theyare particularly ineasy about the uncertainly over SALT. It is believed here that is the Soviet Union were to feel its security in any way challenged, it would immediately seek to reimpose a monolithic orthodoxy on Eastern Europe.

One influential Hungarian commentator said, "The Soviets would restore all teh old ideological controls, preventing us from pursuing many of our liberal policies. The hard-line faction in our leadership would be strengthened - and the first casualty would be good relations with the United States."