Arturo Cruz, a key political figure who resigned yesterday as ambassador to the United States, typifies the many Nicaraguans who are critical of the Sandinista revolutionary government, but feel they must support it in the face of the increasingly hostile attitude of the Reagan administration.

"It is obvious I am not happy," Cruz said today before leaving for Washington where he will hand over the embassy to his successor, whose name has not yet been announced.

"We have come to the point," Cruz said, "at which every key position in the government should be held by a Sandinista."

Cruz, 58, and a member of the Democratic Conservative Party, described himself as "loyal to the revolution, but after all a dissident."

Cruz's resignation also can be seen as another in a long series of disputes between the Sandinistas and the politically powerful community of professionals and businessmen they need to rebuild the country's shattered economy.

Cruz, who from exile in Washington opposed the dictatorship of Anastasio Somoza, returned to Nicaragua after the Sandinista victory, first as president of the Central Bank and then as a member of the ruling junta. Disillusioned and drained, he left the junta in March to become ambassador temporarily and try to improve the already tense relations between Managua and Washington.

The former ambassador has told friends in recent weeks that he has been frustrated by the critical attitude toward Nicaragua expressed by U.S. officials and the Sandinista government's crackdown on dissent.

At a press conference yesterday and a relaxed interview this morning, Cruz made it clear that while he disagrees with much Sandinista policy, he will do everything in his power to support this government rather than see it fall because of U.S. pressure or violent internal opposition.

In recent months the Sandinistas cracked down on dissent from every quarter, shutting down an opposition newspaper for days at a time, tacitly condoning mob violence against the home of an opposition leader and on Oct. 21 arresting four leaders of the Superior Council of Private Enterprise for their denunciation of the government's Marxist leanings and inflammatory rhetoric. Several members of the small Communist Party, which also issued a statement criticizing the government, were also jailed.

Three of the four arrested business leaders were convicted of making statements damaging to the country's economy and sentenced to seven months in jail. One of them, Enrique Dreyfus, is a friend of Cruz.

"It is obvious that I am not happy to see Enrique Dreyfus in prison, or for that matter, the other gentleman, the communist," Cruz said this morning.

But Cruz says national unity is vital now because of the threats -- economic, political and military -- to the country as a whole. He is not alone.

Even La Prensa, the opposition newspaper temporarily closed five times by the government, today denounced Secretary of State Alexander M. Haig Jr.'s "interfering and threatening attitude" toward the country in an editorial entitled "We Nicaraguans Will Solve the Problems of Nicaraguans."

Alfonso Robelo of the Nicaraguan Democratic Movement and other opposition leaders repeatedly point out that U.S. hostility allows the hard-liners among the Sandinistas to hide their failings by appealing to Nicaraguans' intense nationalism.

To meet the threat seen here from Washington as well as from increasingly hostile neighbors like Honduras, Guatemala and El Salvador, many non-Sandinista political leaders support a major restructuring of the government that would, at least temporarily, give the leftists an even stronger hand.

Rafael Cordova Rivas, the only non-Sandinista on the three-man junta, went so far as to say that in Nicaragua, "we need a dictatorship."

The Sandinistas have maintained a collegial administration for more than two years through their nine-member National Directorate, to which the junta is subordinate. Conflicts within the directorate have led to considerable vacillation and inconsistency.

As one opposition figure said privately, "The greatest concern of my friends on both sides is that leaders of the government feel their authority is not taken seriously. They the Sandinistas feel their own rank and file might even challenge them in due course."

If the Sandinistas felt more secure, such people argue, they would not be inclined to such rash actions as the arrest of opposition figures and the closings of La Prensa.

One senior Nicaraguan official said the Sandinistas want to release the jailed businessmen, but the Sandinistas are "extremely fearful this might be interpreted as weakness and to avoid this they are willing to go to almost any lengths."

The massive military buildup here, increasing dependence on Cuban aid and identification with the Soviet Bloc and Nicaraguan involvement with Salvadoran rebels are all focal points for much of Washington's hostility. While the Sandinistas, with their internationalist doctrine, might have pursued such paths anyway, the fear of being accused of weakness in the face of U.S. pressure contributes to their actions.

"I do not doubt for one moment that we are being provoked by Washington," said one disaffected government official, "but we are falling into the trap. The whole thing is hanging by a very thin thread."