President Reagan dispatched Senate Majority Leader Robert J. Dole (R-Kan.) yesterday with a personal letter for Pope John Paul II to "seek his advice" on the situation in Central America.

At the same time, a church-based group planned a nationwide lobbying effort against Reagan's new approach to aiding anti-government rebels in Nicaragua.

Administration officials said they are not worried by initial negative reaction in Congress and in Nicaragua to the plan, but they will work to build support and will keep the offer open.

Aboard Air Force One with Reagan en route to California, national security affairs adviser Robert C. McFarlane told reporters the letter to the pope did not seek help but "was simply to keep the pope informed and seek his advice."

Reagan gave the letter to Dole to hand-deliver to the pope when he visits Rome on a tour of North Atlantic Treaty Organization facilities. Officials refused to disclose its contents.

Reagan's new plan to win $14 million from Congress for the "contra" rebels would involve a cease-fire in Nicaragua and peace talks to be mediated by the Nicaraguan council of Roman Catholic bishops, who offered months ago to mediate any peace effort. The funds would be used for food, clothing and medicine for the rebels for 60 days of talks and then could be used for arms if a determination is made that the talks are stalled.

Dennis Marker, spokesman for a coalition of pacifist church groups called Pledge of Resistance, said an "active alert" signal went out on a telephone network Thursday to 55,000 people nationwide who have pledged to take part in nonviolent civil disobedience in the event of U.S. military action against Nicaragua.

The alert called for "nationally coordinated legal vigils and phone-ins" of protest to the home district offices of all members of Congress the day after Reagan makes "an anticipated nationally televised speech" boosting his Nicaragua proposal.

"We will tell Congress we would view the passage of the contra aid as a declaration of war against Nicaragua," Marker said. Demonstrators also would read aloud descriptions of alleged contra atrocities, he said.

But McFarlane said Reagan probably will not make a televised speech and instead will lobby the members of Congress one-on-one after he returns to Washington April 14. The Senate is expected to vote on the measure April 17 or 18 and the House a week later.

White House deputy press secretary Robert Sims said the president will speak on behalf of the proposal April 15 at a $250-a-plate Nicaraguan Refugee Fund dinner.

Reagan's participation will be "very, very strong" because he is "very, very determined" not to "break faith" with the contras, McFarlane said. He said the White House had received "half a dozen expressions of support" from senators previously opposed to aiding the contras.

"We have improved the prospects for a favorable outcome. That is not to say the votes are there today" for Reagan's proposal, he said. He provided no names and did not give an assessment of the aid's chances in the House.

If the aid proposal is defeated, new congressional proposals will follow, McFarlane said. "One of the exhilarating qualities of our political process is there is no end to the ways of solving problems with Congress."

Earlier, Secretary of State George P. Shultz said on ABC-TV's "Good Morning America" that he is not disturbed by Nicaragua's vehement rejection of Reagan's proposal. "We will keep the offer on the table," he said. "We hope Nicaragua will think it over a little more carefully."

On the same program, Nicaraguan Foreign Minister Miguel D'Escoto, a Maryknoll priest, said Reagan, in effect, "has said, 'You drop dead or else I kill you.' " Thursday he called the plan "a declaration of war."

At an impromptu news conference before his departure, Reagan said, "They were saying that before they even heard what the plan was. I can understand that. They don't want to give up the cushy spot they have now."

Reagan indicated that he also hopes Nicaragua will change its mind. "Their neighbors are going to begin leaning on them also," he said.

McFarlane said three of the four nations of the Contadora regional peace initiative -- Venezuela, Panama and Colombia -- had been "on the whole very supportive" of Reagan's proposal, while Mexico had not yet responded. Jamaica's prime minister, Edward Seaga, is "skeptical" that Nicaragua will take part, he said.

He said the United States won't pressure other Contadora nations but hopes they will "give it their considerable attention and express their views to Nicaragua and to us."