President Reagan last night made an emotional appeal for $14 million to aid the Nicaraguan rebels he calls "freedom fighters," saying that congressional rejection of this assistance would be "a vote against peace."

In remarks at a fund-raising dinner to aid Nicaraguan refugees, Reagan said there was "growing evidence of . . . brutality" by Nicaragua's Sandinista government, including the jailing and torture of opposition party members.

Reagan's speech kicked off an intense lobbying effort designed to persuade Congress to reverse itself and approve funds for approximately 14,000 rebels who have taken up arms against the leftist Sandinistas.

But as the president began his campaign, the assessment both on Capitol Hill and at the White House was that Congress remains solidly opposed to further aid to the rebels. Opponents of the aid proposal are believed to hold a narrow majority in the Senate and as much as a 35-vote margin in the House.

Leadership aides on both sides of Capitol Hill said that sentiment did not appear to have been changed much over the Easter recess by Reagan's new proposal recasting the assistance as "humanitarian" aid that would be used, at least temporarily, for food and medicine rather than arms.

Last night the president used his strongest language so far in denouncing the Sandinistas and depicting congressional rejection of the aid as an invitation for the spread of communism in Central America.

"I believe that a vote against this aid is more than a rejection of the freedom fighters," Reagan said. "It is a rejection of all the forces of moderation, from the Roman Catholic church to the Contadora countries, which have called for freedom and democracy in Nicaragua. I believe one inevitable outcome of a rejection of this aid would be that it would remove all pressure on the Sandinistas to change."

Continuing with his long-expressed view that the Sandinistas have provided a beachhead for the Soviet Union and Cuba on the mainland of this hemisphere, Reagan said that "to do nothing in Central America is to give the first communist stronghold on the North American continent a green light to spread its poison throughout this free and increasingly democratic hemisphere."

"I truly believe that this not only imperils the United States and its allies but that a vote against this proposal is literally a vote against peace, because it invites the conditions that will lead to more fighting, new wars and new bloodshed," the president continued.

Reagan's speech to the $250- to $500-a-plate dinner culminated a long day of lobbying in which the president met with prominent conservatives and Costa Rican President Luis Alberto Monge, who was in Miami for medical treatment and was flown here aboard an Air Force plane to endorse Reagan's proposal.

After meeting with Monge, Reagan posed for photographs with conservatives in an effort to rally support for his aid request. Those present included Zbigniew Brzezinski, national security affairs adviser to President Jimmy Carter; James R. Schlesinger Jr., defense secretary to President Gerald R. Ford, and Jeane J. Kirkpatrick, Reagan's former ambassador to the United Nations.

At the photo session Reagan refused to answer any questions or comment beyond saying he did not have a head count on Capitol Hill.

However, House Majority Leader James C. Wright Jr. (D-Tex.) said that Reagan's new tactics do not appear to have changed votes in the Democratic-controlled House.

One Democratic official said that the House leadership initially was concerned that Reagan's new proposal would have a big impact, but it "went over like a lead balloon."

The House has voted three times to halt aid to the "contra" rebels. Administration officials have said they are six to 10 votes short of reversing this, but House leadership officials said the proposal appears to be 20 to 35 votes short of approval.

Wright said that House Democrats, led by Chairman Lee H. Hamilton (Ind.) of the Permanent Select Committee on Intelligence and Rep. Michael D. Barnes (Md.), likely will offer "a positive, constructive alternative" to Reagan's proposal.

In the Senate, Minority Leader Robert C. Byrd (D-W.Va.) called on the president to assure Congress that his new plan would not offer an excuse for introducing U.S. troops into Nicaragua. An administration official observed yesterday that Reagan repeatedly has said that he does not plan to send U.S. troops into the conflict.

All sides agree that this week's lobbying campaign will be crucial. Dennis Marker, spokesman for a group called Pledge of Resistance, said Reagan's speech last night was the trigger for "thousands of people" to stage noontime vigils today at the home district offices of all members of Congress. Participants are to read from accounts of alleged rebel atrocities and will demand a vote against the aid request, Marker said.

Reagan last night provided new accounts of Sandinista "brutality" against refugees fleeing Nicaragua's "scorched-earth policy." He said the refugees are "fleeing for their lives from the Sandinista police state."

"They are fleeing from people who are burning down their villages, forcing them into concentration camps and forcing them into military service," Reagan said.