The White House, faced with strong congressional opposition to President Reagan's request for continued military aid to the Nicaraguan contras, has launched a fresh effort to win bipartisan support by floating a new peace initiative that includes terms the Sandinistas previously rejected, administration and congressional sources said yesterday.

The diplomatic initiative has the support of House Speaker Jim Wright (D-Tex.). But it immediately drew bipartisan skepticism on Capitol Hill. Democrats who oppose contra aid expressed concern that Reagan was using the peace plan as a gimmick to win continued assistance, while some Republicans complained that the linkage to more contra aid was not explicit enough.

Reagan plans to meet this morning with the congressional leadership to iron out details of the plan and determine whether he and Congress can reach the "same understandings" about it, an informed administration official said last night.

The plan calls for an immediate cease-fire and sets a Sept. 30 deadline for the Sandinista regime to accept democratic reforms, which the regime has rejected in the past, or face the prospect of continued U.S. financing of the rebels fighting the government of Nicaragua. The plan was unveiled to members of Congress this week, on the eve of a meeting of Central American foreign ministers Thursday and Friday in Guatemala City.

The $100 million in military aid for the contras expires Sept. 30. Presidential spokesman Marlin Fitzwater said yesterday Reagan is considering requesting about $150 million for the next 18 months.

Officials said the new diplomatic initiative was the result of an approach 10 days ago by former representative Tom Loeffler (R-Tex.), now a White House strategist on contra aid, to Wright, who expressed interest in a peace plan that could attract bipartisan support. The plan was touted in private meetings on Capitol Hill over the last two days by Secretry of State George P. Shultz and White House chief of staff Howard H. Baker Jr., according to participants.

A House Democratic source quoted administration officials as saying they lack the votes to sustain the contra aid program and felt any successful peace initiative had to come from Congress.

Support for the rebels on Capitol Hill suffered a serious setback with the disclosures of the Iran-contra affair. But polls show at least a temporary rebound in public opinion after the testimony of Lt. Col. Oliver L. North, who oversaw the clandestine aid network to the contras when Congress had cut off military aid.

In the past, Reagan has repeatedly used the promise of diplomatic initiatives as a prelude to his legislative campaigns for military aid to the contras.

In a late 1984 memo made public during the Iran-contra hearings, Rear Adm. John M. Poindexter, then deputy national security adviser, suggested the administration adopt the approach of negotiating with Nicaragua but never agreeing to a treaty.

Yesterday, many Democrats who have opposed contra aid responded to the new gesture by questioning the president's motives in offering the peace initiative.

"It's one of two things," said Sen. Tom Harkin (D-Iowa). "Either the moderates and cooler heads have finally prevailed in the White House in the approach to Nicaragua, or this is one of the greatest ploys -- one of the best I've ever seen -- to put us in a box so contra aid can move forward."

Sen. Dale Bumpers (D-Ark.) said the plan was a "calculated risk." He said "of course, it's appealing, it's a very positive step . . . but we want to know we're not being set up for Sept. 30 when {contra} aid would become almost mandatory" if the peace plan fails.

A Democratic senator who asked not to be identified said, "The mistrust of the White House in the wake of the Iran-contra hearings is so deep that it would take an all-out selling effort by Baker and Shultz over an extended period to make any significant difference."

House sources said Wright wants Reagan to call off any campaigning for contra aid during the 60-day negotiating period. Senate Majority Leader Robert C. Byrd (D-W.Va.) "wants to support it {the plan} but wants to talk to the president first," associates said.

At the same time, Republicans were apprehensive on grounds that the plan could lead to delays for continued funding of the rebels. Sen. Jesse Helms (N.C.), ranking Republican on the Senate Foreign Relations Committee, predicted Democrats would seek to push back the Sept. 30 deadline. "I can hear the cacophony of voices from the House and Senate: 'We need more time, we need six months.' "

House Minority Leader Robert H. Michel (R-Ill.) is also concerned that the plan does not have strong enough guarantees of continued support for the contras if diplomatic efforts fail, sources said.

Reagan is scheduled today to discuss the situation with the recently restructured umbrella leadership of the various contra groups, and may travel to Capitol Hill later in the day, officials said.

Fitzwater refused to discuss details of the plan, but said Reagan wants to "assure the uninterrupted flow of money" to the contras.

An administration official said later that the peace plan does not explicitly require a commitment from Congress for more contra aid if diplomatic efforts fail by Sept. 30. But the official said it does envision a congressional vote on contra aid at the end of the negotiating period, regardless of how diplomatic efforts turn out. Current law provides for an expedited vote on contra aid after Reagan formally submits the request, expected in September.

The peace plan stems from a similar proposal offered earlier by Costa Rican President Oscar Arias, but attempts to rectify what U.S. officials had criticized as defects in the Arias plan. For example, the new plan, as drafted by Wright, calls for an "immediate cease-fire," which U.S. officials said must include a negotiated cease-fire acceptable to the contras. Also, the officials said, on the same day the cease-fire goes into effect, there would have to be what one official called "significant and simultaneous democratization steps" in Nicaragua, such as restoration of civil liberties, suspension of the emergency law in Nicaragua and establishment of an electoral commission for regular elections.

Reagan has made these demands before to no avail. The Wright version of the peace plan also calls for Nicaragua to stop receiving military aid from the Soviet Union and Cuba.Staff writers Helen Dewar and John M. Goshko and staff researcher Michelle Hall contributed to this report.