Almost half the Democratic members of the House yesterday sharply accused President Reagan of undercutting the Central American peace plan and urged him to abandon his continued quest for additional aid to the Nicaraguan contras.

In a letter to the president signed by 120 members of the House, the Democratic lawmakers said, "It is difficult . . . to escape the conclusion that you and your administration are attempting to scuttle the Central American peace plan in order to justify continued funding for the contras."

The criticism came on the eve of an unusual open caucus of House Democrats set for this morning to hear an address by Costa Rican President Oscar Arias, author of the peace plan was endorsed by the five Central American presidents Aug. 7.

The Central American leaders embraced the Arias plan after rejecting a bipartisan peace initiative proposed by Reagan and House Speaker Jim Wright (D-Tex.)

In their letter, the House Democrats accused Reagan of violating his pledge to help implement the Arias plan. "{A}side from a few pro forma positive statements, your administration has done nothing but criticize the agreement and call for more aid to the contras and a continuation of their ineffective war against the Nicaraguan government," the letter said.

In an interview with U.S. News & World Report 10 days ago, Reagan said that the contras "have to be sustained" until an agreement is reached guaranteeing "complete democratization" in Nicaragua. The president's remarks came shortly after Secretary of State George P. Shultz announced that the administration will seek a new $270 million aid package for the contras.

In their letter, the House Democrats said that "to suggest that more contra aid is the answer flies in the face of reality. Your policy of aiding the contras not only has not worked, it has been counterproductive."

House Majority Leader Thomas S. Foley (D-Wash.), at a breakfast meeting with reporters yesterday, said that if the administration formally submits a new aid request while the peace process is still alive, "it will signal that the administration is not fully supportive of the peace process and it will lose."

"The problem," added Foley, "is that the administration is sending up some confusing and very disturbing signals."

The House Democrats said the administration's recent stance on the peace plan and contra aid "risks placing the U.S. in the role of spoiler" and "undercuts our primary objectives of promoting democracy in Nicaragua and protecting U.S. security concerns which we all share."Staff writer Eric Pianin contributed to this report.