The Reagan administration concluded before Christmas that Haitian leader Jean-Claude Duvalier was losing his grip on power, and it "laid the groundwork" for his departure through a series of messages and signals of no confidence, official sources said yesterday.

Reflecting the basic U.S. judgment, the White House and State Department said in statements issued shortly after Duvalier's departure from Haiti that he could only have remained in power longer "through repression and force" and that he had taken a "correct" course in deciding to leave.

Washington officials insisted that Duvalier's decision to leave was his own and that the decision was in doubt until "very late in the game."

But when the "president-for-life" finally decided to flee, U.S. diplomats made arrangements with France starting at 10 a.m. Thursday to accept Duvalier temporarily and ordered an Air Force transport plane to take him and his family to his temporary refuge in France.

Administration statements, minimizing the U.S. role, left the impression that Duvalier's decision to leave came in "a direct request" to the French ambassador in Port-au-Prince for asylum in France, about 2 p.m. Thursday.

But it was learned in Washington that the State Department asked French officials to grant temporary refuge to Duvalier about four hours before Duvalier's meeting with the French ambassador.

It was considered urgent that Duvalier leave Haiti before the Haitian people began their pre-Lenten carnival festivities yesterday. The presence of many thousands of people in the streets, some of them emboldened by freely flowing alcohol, could have brought the country to a new stage of crisis if tensions had still been soaring over Duvalier's future, a State Department official said.

In this sense, he added, the coming of the Haitian carnival celebration, which lasts through Tuesday, was "an action-forcing event" in the protracted maneuvering over Duvalier's fate.

The dual celebration of Duvalier's departure and the religious festival, amid jockeying for power by important elements of the impoverished republic, was described as posing an immediate and "a major challenge" for the successor leadership, initially a five-member junta headed by the military chief of staff.

In retrospect, the fatal shooting of three teen-agers by security forces last Thanksgiving Day in the provincial town of Gonaives seems to have been the beginning of the end for "Baby Doc" Duvalier.

That event caused the Reagan administration to hold up its plan to make an expected certification to Congress that Haiti is "making progress toward improving the human rights situation." Such a certification was required by Congress as a condition of supplying $26 million in aid to Haiti.

Based on reports from the U.S. Embassy in Port-au-Prince, the administration in Washington made a judgment prior to Christmas that "Duvalier was slipping in a way we had not seen before," an official said yesterday. Moreover, U.S. Ambassador Clayton E. McManaway Jr. reported that the United States was widely seen in Haiti as "propping Duvalier up."

The administration in January began taking steps "to distance ourselves" from Duvalier's rule, according to this account, and the Haitian government was officially informed that the necessary aid certification would not be forthcoming.

With Washington's attitude becoming increasingly evident, "friends of Duvalier" approached U.S. officials during January, and increasingly so as the month wore on, asking whether he should leave.

The informal reply, an official said, was that "it would be a good idea." At the same time, officials at the State Department in Washington and the embassy in Port-au-Prince were speaking directly to Haitian figures interested in replacing Duvalier about the conditions for eventual U.S. support.

The administration's attitude toward Duvalier did not hit the front pages of the American press until the strange incident of Jan. 31 when White House spokesman Larry Speakes erroneously announced that "the government of Haiti had collapsed" and Duvalier had fled the country. The statement was exactly one week early, almost to the hour.

State Department officials said yesterday that the premature announcement was simply a bureaucratic mistake inside the government and its timing a coincidence. They said nothing had been decided -- or arranged -- for a Duvalier departure Jan. 30.

Yesterday, State Department spokesman Bernard Kalb volunteered at a news conference that the United States would consider granting emergency aid to "the new Haitian government" if a request is made.

Haitian political figures have been informed repeatedly in recent weeks, according to State Department sources, that the degree of future U.S. support would be affected by the actions of Haitian authorities in seeking "constructive solutions" there, such as release of political prisoners, freedom of the press, educational opportunities, an end to corruption and a move toward democracy.

"We have an intention to supply support to the extent we are shown reforms and a transition to democracy," a State Department official said.

Del. Walter E. Fauntroy (D-D.C.), chairman of the Haiti committee of the Congressional Black Caucus, who had been active in calling attention to human rights abuses there, called for expansion of food aid.

"We should give the interim government a two-year window during which we should do what we can to help them address the desperate plight of the Haitian people," Fauntroy said.