House Democrats completed negotiations yesterday on a $30 million humanitarian aid package for the Nicaraguan contras that would sever the CIA's role in delivering the assistance but offer the Reagan administration another vote on resuming military aid under certain circumstances.

The complex proposal, the result of delicate and lengthy negotiations among Democratic factions following the House's Feb. 3 rejection of an administration aid request that included lethal assistance, is expected to be put to a vote in the House on Thursday.

Democratic leaders said they think they will win what is expected to be a key test for Speaker Jim Wright (D-Tex.), who promised to develop the alternative in order to get moderates to vote against the administration's package.

Because of the House's 219-to-211 rejection of the administration's aid request three weeks ago, all lethal and nonlethal assistance to the rebels is scheduled to cease at the end of this month.

The Democratic proposal would continue $16 million in purely humanitarian assistance to the contras -- limited to food, clothing, shelter and medical supplies -- through June. That would include $3.64 million per month to the contras, and $360,000 per month earmarked for the Miskito Indians. Another $14 million would be designated for "children's survival assistance" to aid young war victims on both sides of the conflict.

If a cease-fire is achieved in Nicaragua, contra aid would be delivered through "nonpolitical" organizations or relief agencies such as the International Red Cross. Absent a cease-fire, deliveries would be handled by the Defense Department, subject to inspection by the congressional intelligence committees and the General Accounting Office.

The legislation providing the aid would also include an absolute prohibition on further deliveries of previously authorized but undelivered military equipment, but would establish a procedure for a guaranteed vote in late June on another Reagan administration request for military assistance. That would occur only if the House intelligence committee certifies that a cease-fire is not in place and that the contras have acted in good faith during the cease-fire talks now under way in Central America.

Rep. Stephen J. Solarz (D-N.Y.), a liberal opponent of contra aid, predicted that "at least" 100 Democrats who have consistently voted against aid would oppose the leadership proposal, but other Democrats scoffed at that estimate.

"That just doesn't jibe with anything," said Rep. George Miller (D-Calif.), a leading opponent of contra aid who said he and other opponents will vote for the leadership alternative because it is "a clear repudiation of military assistance and an expression that Congress wants to see the peace negotiations go forward."

"Our goal is to find something that's 60 percent acceptable to 52 percent of the members and I think we have a 75 percent chance of doing that," said Rep. Barney Frank (D-Mass.).

Frank, a staunch opponent of any aid to the rebels, said he would support the leadership package for tactical reasons because it would make it harder for the administration to win another battle for military aid that could come during action on appropriations bills this summer.

A key GOP supporter of aid to the rebels fighting the Marxist Sandinista government said the Democratic proposal, on initial examination, appeared to fall short of what is necessary to attract significant support among House GOP members, all but 12 of whom supported Reagan's $36.2 million request.

Rep. Mickey Edwards (R-Okla.) said he objected to the plan because it did not guarantee an expedited congressional vote on another military aid request by the administration and because it discontinued the CIA role in delivering the aid.

"The intelligence committee is not a balanced committee," said Edwards. "It's not exactly heavily loaded with contra supporters."