Foreign Minister Miguel D'Escoto dismissed President Reagan's Nicaraguan initiative today as a "desperate, last-minute move" aimed at getting money from a reluctant Congress rather than ending the guerrilla warfare here.

D'Escoto described as "not only ridiculous but insolent" Reagan's plan for a church-mediated negotiation between Nicaragua's Sandinista rulers and the U.S.-backed rebel forces that have been fighting to overthrow the government here since 1981.

The foreign minister underlined the Sandinista government's longstanding contention that peace can be reached only through negotiations between Nicaragua and the United States. In an interview, he rejected the Reagan administration's portrayal of the rebel movement as an uprising by a broad section of the population unhappy with the five-year-old revolutionary government.

[Meanwhile, Reagan sent a letter to Pope John Paul II seeking his advice on the situation in Central America].

"There is a war on that is being declared, directed and financed by Reagan," D'Escoto said. "We want to put an end to it . . . and we're saying to Reagan, 'Please come to your senses. Come and talk. You have no right to the systematic murder of our people.'

"It makes no sense to talk to the hirelings," D'Escoto said of the rebels. "So why talk to them? They can't decide anything. It's Reagan's war. He can stop it."

"Theoretically," he continued, "I have no problem with a government, even our government, at some point in time entering into a dialogue with people who have taken up arms, . . . if you are really talking about an indigenous group. But this is a totally artificially created group of people at the service of the CIA, and in fact they are soul brothers of the president. They see the interests of the president as their own interests. But those are not the interests nor the dispositions of the infinite majority of the Nicaraguans."

The swift Nicaraguan rejection of Reagan's proposals seemed to indicate that they stand no more chance now than when they were made several times in the past by the rebel movement. The main guerrilla group, the Nicaraguan Democratic Force, or FDN, had made the proposals most recently March 1 in a declaration endorsed by a wide range of exiled political leaders, including Arturo Cruz, a one-time member of the Sandinista junta and later an opposition presidential candidate.

D'Escoto described Reagan's actions as "a change in tactics" that the president was "forced to take" because of assessments by congressional leaders that he had no chance to win approval of his request for $14 million to resume CIA funding for the rebels suspended by Congress since last spring.

"So it's a proposal not to Nicaragua, but to Congress, and it says that you must negotiate at the point of a gun in which at 60 days' time you will drop dead or we will kill you," D'Escoto said.

The struggle for approval of the $14 million in new aid for the rebels has become a crucial battle for the Reagan administration. Congressional approval of the money is viewed by the White House as an essential sign of public American support for the rebels to keep them motivated and fighting and therefore to keep pressure on the Sandinista government.

The rebel attacks also are viewed as important in taking pressure off the U.S.-backed government in El Salvador, which is waging a war against leftist guerrillas who the administration says are supported by Nicaragua.

Congressional approval of the aid also is viewed as crucial by the Honduran armed forces and government, which are providing base camps and logistical support for the FDN forces, estimated at about 12,000 armed fighters. Honduran military officers have said they want the money approved so the United States continues to be identified publicly as the rebels' main backers and so the guerrillas have supplies to operate inside Nicaragua rather than being forced back to their camps in the Honduran hills along the border.

D'Escoto said, "Militarily, in spite of the fact that they have caused great hardship to our economy and killed a lot of people," the rebels had "failed" in their offensives. "It's now four years, and where are they?" he asked.

Asked what he thought Reagan might do if he fails to win congressional support, D'Escoto said, "I myself find it difficult to predict. You can expect anything coming from what appears to be a gut commitment to the cause of removing our government. That is not worthy of the president, much less the president of a great nation."

Nicaraguan officials and other diplomats have speculated that Reagan, if thwarted, could restrict trade with Nicaragua further or perhaps even break diplomatic relations.

Despite the four-year-old U.S. campaign against the Sandinistas, the United States remains this country's largest trading partner, and the Nicaraguan government airline Aeronica flies regularly to Miami.

U.S. officials said they doubted that trade restrictions would be worthwhile because they could cause problems with international trade organizations. Both American and Nicaraguan officials expressed the view that the United States probably would not close its embassy here because this would mean losing a source of intelligence.

Interviews with diplomats from many countries here indicated that the Sandinistas are pursuing a strategy based on the assumption that the United States has three options toward Nicaragua. One of these is direct U.S. military intervention, which is widely viewed among the diplomats as unlikely.

The second option is continued U.S. support for the rebels, often referred to as contras, short for counterrevolutionaries. The Sandinista leadership has described 1985 as the crucial year for ending the rebel threat by military and political action such as rallying opposition in Congress to the $14 million in renewed funding.

That would leave option three, which is the Sandinista goal of direct negotiations with the United States on ending the fighting here. A series of such talks was held in the second half of last year but was suspended by the Reagan administration on the ground that they had proved fruitless.

Sandinista leaders repeatedly have expressed fears that the administration is avoiding negotiations because Reagan's real motive is to overthrow their government, which the president has denounced as Marxist-Leninist and a threat to U.S. security through subversion in neighboring countries. They have pointed to Reagan's news conference statement March 21 that he wants to "remove" the present structure of government here and make the Sandinistas say "uncle."

In an interview, Interior Minister Tomas Borge, the Sandinistas' ideological patriarch, said that despite what he called the "enormous problems" the country faces, "I personally think the revolution is ensured to survive."

"I really believe that revolutions cannot be killed by anybody," he said. "There is only the possibility that they can commit suicide, and that is why it is so important for our enemies to try to divide us."

Nicaragua, he said, has serious economic problems and "the United States can cause us millions of problems, but that's not enough to finish the revolution."