WASHINGTON (AP) _ Rep. Bill Alexander has forced the Pentagon to retreat a step in a running fight over his use of a military plane two weeks ago. But a Republican group is attempting to keep the controversy alive.

Alexander, the fourth-ranking House Democrat, got the Pentagon on Thursday to acknowledge it was wrong in saying he had assured an officer the night before departing for Brazil that four other congressmen would be accompanying him.

On the other hand, an Air Force general said Thursday that Alexander never did tell the service he would be the only congressman traveling, and those responsible for lining up a plane were doing so on the basis of a letter from House Speaker Tip O'Neill saying five congressmen would be on the trip.

And a Republican lobbying group saw it another way.

''We feel the only honorable thing for him to do at this point is to repay the (cost of the trip) to the government,'' asserted Kenneth F. Boehm, the chairman of a Republican lobbying group known as Citizens for Reagan.

Boehm made the statement in announcing his group would launch a radio ad campaign in Alexander's Arkansas district today, urging constituents to demand he repay the expense of the military plane - $56,364.

Alexander charged earlier this week the press ''was being taken for a ride by the Republicans'' and that he was the victim of a conspiracy.

''The Republican National Committee filmed the plane arriving back from Brazil,'' he said. ''It's obvious to me they have a hand in this whole conspiracy and we're seeing some dirty political tricks here.''

The controversy surrounding Alexander's trip began Aug. 18 with the disclosure he was the only congressman traveling aboard the military equivalent of a DC-9. The plane carried Alexander, his daughter and several other aides and officials to Brazil to study alcohol fuel production, then dropped the Alexanders off in Honduras for a vacation.

Air Force sources who requested anonymity complained they were the victims of ''congressional subterfuge,'' because Alexander's plane had been reserved on the basis of two letters from O'Neill - dated July 15 and Aug. 7 - specifying he would lead a delegation of four other congressmen and their wives.

Only one of those congressmen, Rep. Ronnie Flippo, D-Ala., has confirmed he initially told Alexander he would go on the trip. Flippo said he canceled out before the Aug. 1 recess, however.

Brig. Gen. Clifford H. Rees Jr., the chief of the Air Force's Office of Legislative Liaison, said in an interview Thursday the Air Force does not maintain hard and fast rules on how many congressmen must travel together to secure a plane.

''But there is an unwritten congressional rule that says three or four makes sense and that less than that might not,'' he said. The general added the requests from top leaders like O'Neill are relied upon because of the need to deal with many requests, reserve proper planes and make logistical preparations.

A spokesman for O'Neill has said previously the speaker left it up to Alexander to make arrangements for the trip. Alexander has said the Air Force knew a week in advance he would be the only congressman traveling.

Rees, whose office handles approved congressional trips, disputed that point on Thursday.

''We had not been told unequivocably (by Alexander) that he was the only congressman going,'' the general said.

Rees also confirmed the Air Force had been ''using the speaker's letters saying there would be a delegation of five as the basis for the (plane) reservation.''

By two or three days before the trip, however, when no other congressman had provided information to obtain visas, Alexander's assigned escort suspected some or all of the other four might not be coming, Rees said.

The escort officer, Maj. Gordy Bendick, was told to doublecheck with Alexander on the passenger manifest, said Col. Peter Sloan, an Air Force spokesman.

''He was told by the congressman not to worry because, 'I'll have the appropriate people there,''' Sloan added.

''I talked to Maj. Bendick after he talked to Mr. Alexander,'' Rees said. ''He recounted the conversation and asked me what to do if other people showed up? And I told him to make arrangements at the first stop to pick up extra visas if that happened.

''The reason Gordy went over there was to tell Alexander that he hadn't gotten anything from the other congressmen and just to make sure that Alexander knew that. And yes, we wanted to be sure, too.''

Alexander has acknowledged he knew on Aug. 6 that none of the other congressmen would be accompanying him. He also maintains, however, that O'Neill's letter of the next day was merely a list of those congressmen ''authorized'' to travel and not meant to be a list of those who would actually do so.