WASHINGTON (AP) _ Nine Republican Congressmen left today on a hastily arranged trip to Nicaragua in the latest surprising development of the battle over President Reagan's request for U.S. military aid to the Nicaraguan rebels.

Two Democratic Congressmen, Reps. Nick Mavroulas of Massachusttes and Marilyn Lloyd of Tennessee, left Thursday night on a similar mission.

As the group departed at 7:45 a.m. from Andrews Air Force Base outside of Washington, Rep. Bob Walker, R-Pa., called the trip ''a learning experience. I've never been to Nicaragua, never had a chance to talk to the Nicaraguan leadership. It can't hurt to learn.''

The Republican group also included Dave Monson of Utah; Paul Henry of Michigan; Jim Kolbe of Arizona; Guy Molinari of New York; Bob Dornan of California; Barbara Vucanovich of Nevada; Thomas DeLay of Texas, and David Dreir of California.

Meanwhile, Pentagon sources, who spoke on condition they not be identified, said that if Congress approves Reagan's request, the Defense Department is prepared to have U.S. military advisers train the Contra rebels, who oppose the Sandinista government of Nicaragua. Such training is now banned.

White House spokesman Larry Speakes said today the administration has ''no specific plans'' to train Nicaraguan guerillas in Honduras. Speakes also said, ''If it were necessary to provide any training to the Contras, the increase in U.S. (personnel) in Honduras would be very, very small.''

Reagan wants to provide the Contras with $30 million in non-lethal aid such as medicine and clothing and $70 million that could be for military aid. Congress last year banned military aid but approved $27 million in non-lethal help that will run out March 31.

Reagan has been lobbying lawmakers all week at the White House and plans to take his case to the public Sunday night in a nationally televised speech.

In a message to Congress today, Reagan said the choice facing lawmakers is whether to ''undercut the president'' and ''betray those struggling against tyranny'' or to join in a bipartisan foreign policy effort.

Speaking today to about 200 local elected officials from around the country who support his aid program, the president criticized suggestions that the military aid be delayed while efforts are made to negotiate a settlement.

''If we delay aid for a few months while we are talking, the Sandinistas will take the time and use it to finish off the Contras,'' he said. ''That's the communist strategy - to kill them off. And when the execution is complete they will end the talks.''

Dornan said he and his colleagues were going on what he called ''a trip of hope, a last-minute trip to get a breakthrough'' which he said could either lead to peace or to strong U.S. support for the rebels.

DeLay told reporters at the airport, ''At least we're talking to our opposition, unlike the Sandinistas government of Nicaragua.''

The Democratic-controlled House is now scheduled to vote March 20 on the package, and both Democrats and Republicans say Reagan is facing an uphill fight. The Republican-run Senate will vote later.

Kolbe said he is undecided on how to vote in the House next Wednesday. ''I felt this was a useful way of gathering information. I won't become an expert in 36 hours, but I will gain information. I think it's a useful trip.''

Molinari voiced similar sentiments, ''I think this will be a learning experience.''

Planning for the two-day trip began Wednesday, Dornan said. It will include a visit to the capital city of Managua and then a flight tomorrow to neighboring El Salvador, where the U.S.-backed government is fighting guerrillas the Reagan administration says are supplied by the leftist Sandinistas.

The legislators, accompanied by State Department officials, will meet with Salvadoran President Jose Napolean Duarte and will also meet with government and Roman Catholic church leaders in both countries.

Speakes said Reagan had not requested the trip.

''We just got a request for a plane,'' said Speakes. ''They said they wanted to go and we said okay.''

Mrs. Vucanovich, in a telephone interview, said: ''I am a supporter of the president's program. But I have had a lot of people come to see me from Nevada and call me and they say they feel a lot of us were simply voting with the president without knowing the situation down there. We're going down to see for ourselves, to meet with people on both sides.''

The House vote had originally been scheduled March 19, but has been moved back a day at the request of House Minority Leader Robert Michel, R-Ill., said the congressional sources. The House will begin formal consideration of the aid package Wednesday, but there are 10 hours of debate scheduled before the vote. In lobbying for the aid package, Reagan and top administration officials have refused to spell out exactly how the $70 million will be spent.

But the Pentagon sources said that in addition to U.S. military trainers, the administration might also share U.S.-gathered intelligence with the rebels about the location of Nicaraguan forces.

House Speaker Thomas P. O'Neill Jr., D-Mass., said he was fighting Contra aid because ''I still have a terrible feeling that the policies of the president are ultimately going to bring us into that war. That's what I'm fighting for, to keep our boys out of there.''

The New York Times reported that Reagan plans to send to Congress today a policy statement saying that his administration will oppose ''tyranny in whatever form,'' including dictatorships of the anti-Communist right as well as the pro-Soviet left.

In today's editions, the newspaper quoted Reagan administration officials as saying the statement will lay out a regional security strategy, and that a section supporting human rights is intended to gain congressional support for covert military aid to the Contras and other anti-Soviet ''freedom fighters.''

Reagan visited the State Department Thursday to view a cache of U.S.-made weapons the administration says were captured from the anti-government rebels in El Salvador.

''These weapons, and the testimony that we have heard, demonstrate the magnitude of a sophisticated communist effort to undermine democracy in this hemisphere and to deceive us in the process,'' he said.