Sergio Ramirez, the Sandinista vice presidential candidate, was explaining recently what he said was Nicaraguans' willingness to endure sacrifice to further the cause of their five-year-old revolution.

"He who is used to living standing on his feet is not going to get down on his knees to pick up toothpaste," he declared.

The remark was easily understandable to Nicaraguans here in the capital and around the country. Partly because of U.S. economic pressure and backing for insurgents, the Sandinista leadership has encountered such severe foreign-exchange problems that toothpaste often is unavailable.

The defiance reflects the attitude of Sandinista leaders and many of their followers, who are encouraging a mood of national awakening after years of seeing their country take its lead from Washington during the rule of the Somoza family. Although popular support for the Sandinistas has dropped markedly during the past year, a core of Sandinista militants and thousands of Nicaraguans participating in and benefiting from the revolution have remained committed to transformation of the country whatever the price.

"There is a cohesive factor, which is dignity," said Gioconda Belli, a militant poet who heads the Sandinista election campaign. "We have recovered a feeling of nationality that we did not have before. We have once again become the owners of the country."

Belli is one of approximately 15,000 members of the Sandinista National Liberation Front, which spearheaded the overthrow of the late Anastasio Somoza in July 1979 after 18 years of clandestine struggle and guerrilla warfare. Although Nicaragua is administered by the junta's National Reconstruction Government, the Sandinista Front's nine-member National Directorate has made all the major decisions since seizing and consolidating its hold on power.

The Reagan administration has characterized this as a classic march into Cuban-style Marxist-Leninist dictatorship. Viewed on the scene, however, "the process," as the Sandinistas call it, has resembled more a kaleidoscopic mixture of Marxism, nationalism, Latin bravura, youthful zest and social reform, with shifting patterns depending on which faction of the nine-member Directorate has prevailed on the problem in view.

In addition to its own membership, the front has organized a nationwide network of neighborhood militant groups called Sandinista Defense Committees, CDS by their Spanish initials. In doing so, the Sandinistas followed a precedent set by Cuba, with its Revolutionary Defense Committees, and U.S. big-city political machines, with their ward systems.

Tomas Borge, interior minister and one of the few survivors of the founders of the Sandinista Front, estimated this month that 800,000 Nicaraguans have enrolled in Sandinista Defense Committees in cities, towns and villages around the country. With more than half of Nicaragua's 3 million inhabitants teen-agers or younger, that would amount to a majority of the adult population.

In addition, the Popular Sandinista Army, which assigns political officers to help guide its units, has by U.S. count grown to more than 50,000, including militias, with a target of 30,000 more youths being drafted now for two-year terms. And the Sandinista Youth, an organization aimed at secondary and university students, has enrolled about 30,000 members, its leaders reported.

But Nelson Dona, a 23-year-old CDS block leader in a poor Managua neighborhood, explained that enrollment does not always mean participation. The 25 families in his block of 15 houses total about 150 persons, he said, but they have organized "revolutionary vigilance" for only six days a week because not enough people want to stay up all night guarding against what Dona called "antisocial behavior."

With his CDS leadership, Dona has received the right to bestow or refuse letters of recommendation that Nicaraguans frequently need for such things as government jobs, driver's licenses or visas.

The letters are supposed to reflect revolutionary zeal as measured by participation in CDS meetings, revolutionary vigilance, Sandinista rallies, coffee-picking brigades and the draft, Dona said. But persuasion is better than coercion, he added, and Dona writes whatever letters his neighbors need, no matter whether they have crossed the Sandinista zeal threshold.

"I try to show the youths that it is not a question of obligating them just for the sake of obligating them, but that it is because of the situation the country is going through," he said in a recent conversation with foreign visitors to his home.

Dona, who was studying in Mexico in July 1979, has been participating in the revolution since volunteering to teach in a literacy campaign in rugged North Zelaya province on his return to Nicaragua that September. He said "the process" is necessary to rid the country of "capitalist dependency" on the United States and provide better lives for Nicaraguans in the long run.

The Sandinista Front "has its faults," Dona said. "It has its errors. But the Sandinistas have answered the needs that the Nicaraguan people have expressed. We could not say the front is the only right one answer , but it is the most promising."

Dona, who works in an engineering firm by day and studies architecture by night, said his main effort recently has been to ensure that the neighborhood government store has enough powdered milk, beans and rice. Scarcities have arisen, he said, because of U.S. aggression and economic isolation.

"If the Sandinista Front did not have these limits, this economic blockade, the situation would be much better," he said.

A merchant selling cheese and vegetables in Leon's covered market agreed, blaming the Reagan administration for her paltry wares. "It is not the fault of the government," she said. "It is because of the situation they are imposing on us, because of the aggression."

Despite her remarks, the woman added that she has avoided joining her neighborhood Sandinista Defense Committee and a similar group in the market. Although Sandinista police patrolled the stalls, many of her colleagues expressed bitter disillusionment to foreigners who strolled through the busy aisles one recent morning.

Although their complaints seemed clearly to weight public opinion against the Sandinistas, in the Leon marketplace at least, militants have remained confident that the momentum of their revolution, combined with nationalist reaction against the United States, can carry a majority of the country through the disappointment the way it has carried Nelson Dona and his family.

"Not everybody is Sandinista, but we are the majority," said a young Planning Ministry official returning to Managua from a visit to his girlfriend in Esteli. "And even those who are not Sandinistas at bottom are Sandinistas, because their differences are not about the fundamental principles."

His mother used to complain mightily about shortages in the grocery stores, he said by way of example, but he has explained "the situation" and "now she understands."