As President Reagan toured Grenada to push his foreign policy objectives, House Democratic leaders served notice yesterday that they will strongly oppose two of Reagan's top priorities: military aid to the rebels fighting the Sandinista government of Nicaragua and covert aid to the forces opposing the Marxist government of Angola.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) predicted that congressional approval of a $100 million military and economic aid package to the Nicaraguan counterrevolutionaries, also known as contras, as sought by the administration would be followed by "disaster" and the "slaughter" of the rebels by government forces. Eventually, he said, that would lead to the dispatch of U.S. troops to that Central American country.

"Give $100 million and our boys would be in there," O'Neill said. "That is what I'm fighting against all the way."

Meanwhile, House Majority Leader James C. Wright Jr. (D-Tex.) said that if the administration insists on providing covert military aid to the UNITA rebels of Jonas Savimbi in Angola, against the advice of the chairmen of the House and Senate intelligence committees, it will be setting up a "confrontation" with Congress with possibly unintended results.

Wright suggested that Congress might attempt to revive versions of the Clark amendment, which banned covert aid to UNITA before it was rescinded last July, and the Boland amendment, which prohibited aid to the Nicaraguan rebels before it expired last October. The amendments are named for their chief sponsors, then-Sen. Dick Clark (D-Iowa) and Rep. Edward P. Boland (D-Mass.).

Such a move by the House would likely run into strong opposition in the Republican-controlled Senate and an almost certain presidential veto. But the comments of O'Neill and Wright indicated the depth of House opposition to many of Reagan's foreign policy goals, particularly in Central America.

The Democratic leaders also brushed aside administration suggestions that Congress take a cautious approach toward the future of Philippine President Ferdinand Marcos, setting up the possibility of a three-way confrontation with the administration over foreign policy issues. O'Neill said he will bring to the House floor "forthwith" any legislation designed to pressure Marcos to resign by cutting off or restricting U.S. aid.

O'Neill, who also said he is "absolutely opposed to covert aid to Savimbi," whom he characterized as "an agent of South Africa," made the remarks after a House Democratic leadership meeting. They came on the day that Reagan toured Grenada, scene of an October 1983 U.S. invasion, where he called for support for the Nicaraguan rebels and other anti-communist forces around the world.

"I can see a big Hollywood showdown there," O'Neill said of the presidential visit. "This is the start of a kickoff by the right wing for American funding [of rebel forces] in Angola and Nicaragua."

Congress is in a much stronger position to block the Nicaraguan aid package than it is to halt administration plans to supply covert aid to Savimbi. The administration has notified the two intelligence committees that it plans to supply Savimbi's forces with $10 million to $15 million. Earlier this week, Assistant Secretary of State Chester A. Crocker told a Senate committee that this process is "in motion."

The administration has the funds available to aid Savimbi, and Congress could block their use only by reenactment of the Clark amendment or similar legislation.

In the case of the Nicaraguan rebels, however, the administration is expected to make a formal request soon for authorization of a $100 million package made up of $70 million in covert military aid and $30 million in overt, nonlethal assistance such as clothing and medical supplies.

Under a procedure adopted last year when Congress reluctantly authorized $27 million in nonlethal aid to the contras, the new aid request cannot be ignored but must be brought to the House and Senate floors for a vote.

Rep. Michael D. Barnes (D-Md.), chairman of the House Foreign Affairs subcommittee that deals with Central America and a strong opponent of aid to the Nicaraguan rebels, said his panel will explore "all possibilities," including a revival of the Boland amendment, in an attempt to halt the aid package.

"I think the president's request will run into very strong opposition in the committee and the Congress," Barnes said. "Whether or not there are the votes to reinstate the Boland amendment, I don't know."

Warnings about the shaky prospects for the Nicaraguan aid package were also voiced yesterday by Senate Majority Leader Robert J. Dole (R-Kan.) and House Minority Leader Robert H. Michel (R-Ill.). Because of budgetary constraints, the two GOP leaders said the $100 million should be "reprogrammed" from other foreign aid accounts and not involve new money.