SANTA BARBARA, CALIF., AUG. 20 -- President Reagan, in an effort to reassure U.S. conservatives that he is not abandoning the Nicaraguan contras, will meet for the first time next week with the contras' top military leader, White House spokesman Marlin Fitzwater said today.

Fitzwater said Reagan will meet Thursday in Los Angeles with the six political leaders of the contra directorate and also with Enrique Bermudez, military leader on the northern front in Nicaragua, where the bulk of contra troops are concentrated.

"Make no mistake about it," Fitzwater said at a briefing here. "We want to demonstrate to the conservative leaders and to the directorate and to the nation that the president will not desert the contras. That is a purpose of the meeting, absolutely. But beyond that, there are are a lot of substantive things to talk about in terms of the status of the peace process, and {in} demonstrating our commitment to the contras . . . . "

Reagan has met many times with members of the directorate, the political leaders of the Nicaraguan resistance movement and has, throughout his presidency, expressed support for contras, whom he usually calls "the freedom fighters."

But administration officials said Reagan has never met with Bermudez, a junior officer in the National Guard during the regime of deposed dictator Anastasio Somoza, who was overthrown by the leftist Sandinistas in 1979. To the Nicaraguan rulers and to many liberals outside the country, Bermudez remains a symbol of Somoza's despotism.

Asked whether meeting with Bermudez will send "the wrong political signal" during Nicaraguan peace talks, Fitzwater replied: "We simply want to get an update on the military situation and where the resistance stands in terms of their activities and operations, and he's the best man to do it."

Alfredo Cesar, one of the contra leaders who will meet with Reagan next week, told the Associated Press in an interview from Costa Rica on Wednesday that he and other contra leaders doubt the sincerity of Nicaraguan President Daniel Ortega in signing the peace plan worked out Aug. 7 by the presidents of Costa Rica, El Salvador, Guatemala, Honduras and Nicaragua. But Cesar said the contra leaders are willing to give the plan a chance.

The contras say that they are worried that if the administration takes no action on renewing aid to their forces, there will be no incentive for Nicaragua's ruling Sandinistas to live up to their agreements under the plan. Fitzwater said today that the Reagan administration stands by its commitment to House Speaker Jim Wright (D-Tex.) not to submit a contra-aid request before Sept. 30, "if then."

White House officials have acknowledged privately that it will be very difficult to win congressional approval of any military-aid request if the Sandinistas show any signs by Sept. 30 of democratizing their regime.

The Los Angeles meeting will aim at coordinating administration and contra positions on the new Central American peace plan, Cesar said.

Meanwhile, senior White House officials convened here for a planning session chaired by chief of staff Howard H. Baker Jr. A senior official said the aides explored ways to "get the president back on the offensive and keep him there" during the remaining 17 months of his term.

Baker is said to be emphasizing the importance of a superpower arms-control agreement that would remove U.S. and Soviet medium-range and short-range missiles from Europe and Asia. The president will address this issue during a Los Angeles speech Wednesday on U.S.-Soviet relations.

High on the president's domestic agenda is confirmation of his Supreme Court nominee, Judge Robert H. Bork. Fitzwater said Reagan will meet in Los Angeles with Bork supporters on Aug. 28 before returning to the ranch the next day to continue his vacation.