The chairman of a Democratic congressional delegation on a fact-finding mission in Nicaragua said today that any financial aid package to the anti-Sandinista guerrillas will have to provide for U.S. military advisers.

"I am convinced that if contra aid is passed in the Congress, we will have to provide U.S. advisers and trainers to work with the freedom fighters in the use of this military equipment," said Rep. G.V. (Sonny) Montgomery (D-Miss.), a member of the Armed Services Committee, who voted in favor of the Reagan administration's proposal for a $100 million in aid to the contras. President Reagan has referred to the contras, or counterrevolutionaries, as freedom fighters.

"I know you say that starts another Vietnam-era process," he said at a news conference in Managua. "I can't buy that. I think we've learned something from Vietnam." Montgomery said advisers would not be in combat roles inside Nicaragua. He did not elaborate, but a western observer here said, "It's clear what the administration has in mind is a training program in Honduras: officer training, NCO training, special weapons training, logistics training."

In Washington Sunday, Secretary of Defense Caspar W. Weinberger said on NBC-TV's Meet the Press that there are no U.S. advisers with the contras at their bases in Honduras and that any military trainers who might be sent as part of an aid package "would certainly not operate anywhere in Nicaragua." He noted that there are fewer than 55 U.S. military trainers in El Salvador and indicated that any advisers sent to train the contras would number no more than 50.

In a speech yesterday, Nicaraguan President Daniel Ortega said this year's proposal for $100 million in aid -- several times the amount proposed last year -- indicated the contras' "capacity to fight against our mobilized people, and our Army is extremely limited. If the U.S. sends 12 advisers or a hundred advisers . . . the contras will nevertheless be destroyed."

Congress voted Thursday 222 to 210 against the Reagan administration's proposal for $70 million in military aid and $30 million in nonlethal assistance, despite the president's strong lobbying effort.

The Democratic congressmen, who ended a two-day visit today, said they had come to gather information before voting on "alternative" contra aid bills expected in April. The other members of the delegation were Reps. Kenneth Gray (Ill.), Sander M. Levin (Mich.) and George Darden (Ga.).

Earlier this month, a delegation of nine Republican congressmen visited Managua under White House sponsorship.

Like Montgomery, Darden suported the aid package, while Gray and Levin voted against it. Gray and Levin disagreed with Montgomery's statement that aid to the contras would require U.S. military advisers and called for negotiations sponsored by the Contadora group of Latin American nations seeking a regional peace agreement.

During their visit, they met leaders of Nicaragua's domestic opposition, members of religious organizations considered sympathetic to the government and with Nicaraguan Vice President Sergio Ramirez.

Ramirez said he warned them about "the danger of falling into paranoia." He said Reagan's recent speech promoting the $100 million proposal contained "paranoid elements" and "complete lies," such as a charge that the Nicaraguan government firebombed a synagogue.