Rejecting a last-minute plea by Secretary of State George P. Shultz, the House Democratic leadership yesterday adopted a key parliamentary tactic that the Reagan administration charges could effectively scuttle its proposed $100 million aid package for the Nicaraguan rebels.

At a half-hour meeting with House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), Shultz urged that next week's scheduled House vote on aid to the counterrevolutionary rebels, known as contras, be separated from consideration of the fiscal 1986 supplemental appropriations bill.

But O'Neill, according to his aides, rejected this, and several hours later the House Rules Committee approved the controversial ground rules tying the contra aid package to the appropriations measure when both bills reach the House floor next week.

Those ground rules, if adopted by the full House in what may be the key showdown in the legislative battle, would automatically incorporate the contra aid package into the spending bill if both measures are approved by the House. The administration has strong objections to several provisions of the $1.7 billion appropriations bill, and White House officials yesterday threatened a presidential veto even if that kills the contra aid at the same time.

The administration also argues that the contras are likely to be crushed by Nicaragua's Sandinista government in the weeks or months it will take for the appropriations bill to clear the Senate and finally be enacted.

House Republicans charged yesterday that by linking the two, Democratic opponents of administration policy in Central America were attempting to undermine that pro-contra policy by parliamentary maneuver.

"It's a sham and a fraud," said Rep. Dick Cheney (R-Wyo.), the chairman of the House Republican Policy Committee. "The bottom line in this exercise is that this is a situation where they are trying to do by parliamentary device what they don't have the votes to do on the House floor."

Cheney added: "The Speaker might as well take to the floor and say, 'I support the Sandinistas, I support communism in Central America.' "

"It's blackmail, pure and simple," said Rep. Henry J. Hyde (R-Ill.). "It shows that they are unserious about the important issue of the survival of democracy in Central America."

The House Democratic leadership promised a second vote on the contra aid issue last month, when the House initially rejected the administration proposal by a vote of 222 to 210. The Democrats argue that the appropriations bill is the best vehicle for this because it is almost certain to be enacted by Congress.

Yesterday's preliminary skirmishing came against the backdrop of the controversy that erupted Wednesday with the leak of a classified State Department cable that criticized the conduct of O'Neill and other members of a congressional delegation during a visit to South America last month.

In the cable, which was made available to The Washington Post, the U.S. ambassador to Argentina, Frank V. Ortiz Jr., accused O'Neill and others of attempting to pressure political leaders in Buenos Aires to condemn Reagan administration policy in Central America.

Several members of the bipartisan delegation retorted with suggestions that it was Ortiz's behavior that was "out of line" during the meetings with Argentine leaders.

The Ortiz cable apparently came up during Shultz's conversation with O'Neill, although neither man would discuss it. At the State Department, spokesman Bernard Kalb expressed "regret" over the leak of the cable but defended Ortiz, who he said "continues to enjoy the confidence of this administration."

In his first reaction, O'Neill said the congressional delegation "didn't get the normal treatment and courtesies" from the U.S. Embassy during its stop in Argentina. He said Ortiz was "kind of resentful we were there," adding: "There was a rudeness about him, to be truthful."

Meanwhile, Rep. Marty Russo (D-Ill.), a member of the delegation who Ortiz charged had "exceeded all bounds in his attacks on the president," hosted a showing in his office of what were quickly dubbed on Capitol Hill as the "Russo tapes." This was a videotape that Russo shot while the delegation met with Argentine President Raul Alfonsin showing one instance in which Ortiz interrupted the conversation and was rebuked by Rep. Fortney H. (Pete) Stark Jr. (D-Calif.).

House Minority Leader Robert H. Michel (R-Ill.) insisted yesterday that Republicans have picked up enough votes to reverse the House's March 20 rejection of the contra aid package "if we had a clean shot at it," unencumbered by the supplemental appropriations bill.

The ground rules set by the Rules Committee allow the White House only one way to achieve victory: passing the contra bill and defeating the spending bill. There were conflicting assessments yesterday of the chances of doing this, but the supplemental appropriations bill contains a number of provisions that are important to various lawmakers. One of these is of particular interest to O'Neill -- the first $50 million appropriation in a planned $250 million package of economic aid to Northern Ireland.

"Who's going to vote against aid to Ireland?" asked Michel.

The debate and votes, first on the contra aid package and then the appropriations bill, are now set for next Tuesday and Wednesday.

Under the proposed ground rules, House Republican leaders will offer a slightly modified version of the contra aid package that was narrowly approved by the Senate last month. This would immediately provide $25 million in aid to the contras, which could be used for military training and the purchase of "defensive" weapons, including surface-to-air missiles.

Release of the remaining $75 million would be delayed until mid-July while the United States sought negotiations for a peaceful settlement. The funds would be automatically provided then if the president certified that negotiations had failed, although Congress could still halt the aid by passing a resolution of disapproval. Such a resolution, however, would be subject to a veto.

Rep. Lee H. Hamilton (D-Ind.) will offer a second plan, providing $27 million to aid Nicaraguan refugees and $2 million to the so-called Contadora group of Latin American nations that are seeking a peaceful settlement. It would provide no funds to the contra rebels.

A third alternative, to be offered as an amendment to the Republican plan, will be sponsored by Rep. Dave McCurdy (D-Okla.). This proposal differs from the Senate-passed contra aid package in that it would provide $30 million in aid immediately, but prohibit its use for the purchase of weapons. It would also require a second, affirmative vote by Congress to release the remaining $70 million if negotiation efforts had not succeeded by the end of July.