SAN SALVADOR, SEPT. 9 -- Rep. Jack Kemp carried the conservative cause through Central America this week at the head of a 65-member delegation, called "Mission Liberty," that seeks $310 million in U.S. aid for Nicaraguan rebels.

The New York Republican found a ready adherent here last night in Salvadoran ex-major and fellow presidential candidate Roberto d'Aubuisson, who said he hoped Kemp would "come to power soon . . . because we both need each other." D'Aubuisson, a leader of the right-wing Arena party, has been linked to death-squad activity in his country.

Kemp, hopping through four countries in two days, sang the praises of freedom, democracy and free enterprise while selling presidents and guerrillas alike on the 18-month aid package he is cosponsoring with Sen. Jesse Helms (R-N.C.). The money would continue funding the rebels after the current $100 million appropriation expires Oct. 1.

Kemp made no effort to be diplomatic about his feelings for the regional peace plan signed last month in Guatemala City by the five presidents of the region. A two-hour meeting with Salvadoran President Jose Napoleon Duarte was described by an observer as heated.

"I disagreed with President Duarte that that piece of paper is going to be enough to bring democracy to Nicaragua," Kemp said.

Kemp's people found out quickly that what sounded good in Washington was more difficult to put into practice in Central America.

The group had hoped to spend two nights in San Jose, Costa Rica, using it as a base to fly to Honduras and El Salvador for meetings with the contras, as the rebels are known, and the presidents of those two countries.

Mission Liberty was stymied, however, by the presence of contra leader Adolfo Calero, who flew with the group from Washington. The Costa Rican government refuses to allow Calero in, so the mission was forced to spend Monday night in Guatemala. Calero stayed in Honduras yesterday and the delegation flew to El Salvador and Costa Rica without him.

Central American sensitivities also scrambled Kemp's effort to reap a publicity harvest by visiting with anti-Sandinista leaders at their Honduran bases. Reporters on the plane had been promised they would be able to accompany the congressman.

But the press tour went the way of a similar effort by Sen. Robert J. Dole (R-Kan.) during his campaign swing through the area last week. The reporters spent the day visiting a Nicaraguan refugee camp while Kemp flew to the officially unacknowledged contra base with Calero. Unlike Dole, Kemp avoided Nicaragua.

Kemp did keep the Honduran secret about the base. After meeting with Honduran President Jose Azcona, Kemp told reporters in Tegucigalpa he had visited the rebels at a secret location "somewhere in Central America."

The reality of Central America seemed to surprise many of Kemp's aides. After getting off their chartered jet at Ilopango Air Force Base in San Salvador, many were in awe as gun-toting guards wearing bullet-proof vests protected the buses they rode in.

At their hotel, mission members received a briefing book from the U.S. Embassy instructing them to stay away from the windows should they hear shooting or explosions, "a regular occurrence" in San Salvador.