The House Democratic leadership yesterday invited President Reagan to join in fashioning a bipartisan package of humanitarian aid for Nicaragua's contra rebels.

But the White House, still undecided about its next move to help the contras, diplomatically held the Democrats' offer at arm's length.

This round of fencing over the contra-aid issue followed the House's rejection last week of Reagan's request for $36.2 million in military and nonlethal assistance for the rebels fighting Nicaragua's Marxist Sandinista government.

House Speaker Jim Wright (D-Tex.) had promised wavering members that if the administration request were defeated, the Democratic leadership would offer an alternative proposal for purely humanitarian aid. Yesterday Wright, joined by other members of the Democratic leadership, wrote Reagan and Secretary of State George P. Shultz, inviting them to participate in putting together an aid plan that Wright hopes to offer by the end of the month.

"Rather than preparing for a confrontation at the conclusion of the process surely it would be much better to work together at the outset in perfecting the program," the letter said.

In response, White House spokesman Marlin Fitzwater said the administration is willing to work with Wright "on a bipartisan basis." Other officials said Reagan would send a formal reply to the speaker in a day or two.

But Fitzwater summed up the basic White House view when he said, "We want to see what their proposal is before we make a commitment to support it. They rejected our plan, and now the American people have a right to see what they offer the resistance."

Fitzwater's comment came after a White House meeting at which administration officials reportedly decided not to accept Wright's offer at this time. Administration sources said the feeling was that to do so might foreclose Reagan's lingering hope of finding a way to keep the contras able to put military pressure on the Sandinistas.

One senior official called the letter an attempt to "get the White House a little bit pregnant" by entangling it in a commitment to support a package limited to humanitarian aid, which the Democrats define as food, clothing, medicine and shelter.

The administration view is that for now it wants to reserve the right to try to amend the House humanitarian aid proposal when it reaches the Senate, which last week voted in favor of the administration request. Administration officials hope the Senate might add some military assistance or "dual use" items such as helicopters and communications equipment that have military uses.

The Democrats also want to halt the past practice of delivering supplies to the contras through airdrops managed by the Central Intelligence Agency. But the administration argues that other suggested delivery means -- among them, third countries, international organizations such as the Red Cross or groups such as the Nicaraguan Bishops Conference -- cannot do the job.

Wright and Democratic Whip Tony Coelho (D-Calif.) also warned yesterday that even prospects for humanitarian aid could be harmed if the contras persist in putting off cease-fire talks with the Sandinistas and pursue plans, announced at a Miami news conference Monday, to seek aid from other countries and private U.S. citizens.

The contras were scheduled to resume meeting with the Sandinistas today in San Jose, Costa Rica, but announced that they were postponing the talks to consider their bargaining strategy in the wake of the House vote. A Sandinista adviser told United Press International that the goverment has asked for an "urgent" meeting with the contras this week to discuss the agenda for future negotiations.

Wright said that because the contras are regarded as surrogates of the administration, the contra move could be an administration-orchestrated attempt to stall movement on implementing the five-nation Central America peace agreement.

"I hope the administration will act swiftly and decisively to urge the contras back to the negotiating table," Wright said.

Coelho, recalling the Iran-contra scandal's revelations of how the administration had used private sources to secretly send aid to the contras in the past, charged that the contras' aid solicitation plans sounded suspiciously like a reversion to such tactics.

"They're trying to do publicly what they tried to do privately before," he asserted. "I think there's a deliberate attempt by the administration to stop the peace process."

"But we are assured that they intend to continue the cease-fire talks and that they will resume again as soon as possible," Fitzwater said.

Fitzwater said that Reagan made it "absolutely clear" that the administration will not become involved in fund-raising for the contras.

He said, "We're very sensitive about the Iran-contra business. It's obvious that the administration would be hurt if it became entangled in private fund-raising for the contras, and everyone realizes that."

Staff writer Tom Kenworthy contributed to this report.