House Republican leaders have bluntly warned the White House that the administration's prospective $100 million package of military and economic assistance to Nicaraguan rebels is in jeopardy.

White House officials said the warning from House Minority Leader Robert H. Michel (R-Ill.) and Rep. Dick Cheney (R-Wyo.) has prompted a reappraisal of administration strategy and could mean reducing the amount of the aid request before it is formally presented to Congress. The warning appeared to give pause to some administration officials who as recently as a week ago thought that a substantial aid program to the rebels was a virtual certainty.

But these high-ranking officials reiterated that, regardless of the odds, President Reagan is determined to push forward with some sort of package to help the contras, or counterrevolutionaries, fighting the leftist Sandinista government in Nicaragua. "The Nicaraguan freedom fighter image is second with the president only to the American hero image," one Republican strategist said.

Michel and Cheney visited the White House last week and told White House officials that they would have a very difficult time winning any request for military aid. At the same time, the White House received a letter from 31 House Democrats who voted for humanitarian aid last year. They urged Reagan to postpone his request for more aid until a final regional peace effort could take place.

One White House official said the combination of the visit from the Republican leaders and the letter from the Democrats had the effect of "injecting a note of reality" into administration discussions.

"Expectations are too high," said a senior official, who hinted that the total of the aid request may have to be lowered. "We may have to take a step back before we can take two steps forward."

White House officials said meetings are planned this week to hammer out the strategy and details of the aid request, including its amount.

The administration also must determine whether to seek overt or covert military aid for the rebels. "There's very strong sentiment on the Hill against covert aid because Congress just doesn't trust the CIA," one official said yesterday.

Present plans call for Reagan to kick off the campaign for the aid package during a visit to Grenada on Feb. 20. White House officials said they intend to consult with congressional leaders that week on the composition of the aid package, which they would like to submit to Congress the week of Feb. 24.

Last year, after Congress scuttled the administration's program for military aid to the anti-Sandinista rebels, the White House was able to win reluctant congressional approval of $27 million in humanitarian aid, which expires March 31. Congress imposed restrictions on this assistance, including provisions that it not be administered through the Central Intelligence Agency or the Defense Department.

But White House officials who savored last year's victory said that in retrospect it might not have occurred except for the late April visit of Nicaraguan President Daniel Ortega to Moscow, which outraged some Capitol Hill defenders of the Sandinista regime.

"It was really the Sandinistas who kept the aid package alive," said one official. "We're not likely to have that kind of help this time."

White House strategists said the Capitol Hill climate has shifted in their favor, but not necessarily enough to win approval of military aid. At the same time, they said, Reagan has become even more convinced that aid to what he calls Nicaragua's "democratic resistance fighters" must include some arms.

"This is really central to administration policy," a senior official said.

Ever since he spoke at the United Nations last October, Reagan has made financial assistance to "freedom fighters" in regional struggles throughout the globe a centerpiece of his policy. These regional conflicts, mentioned again in the president's State of the Union address last Tuesday, include Nicaragua, Afghanistan, Angola and Cambodia.

Political strategists argued successfully with the president a week ago that a $1.5 million proposed arms sale to Jordan should be postponed indefinitely in the face of congressional opposition. One point they made was that persisting with the Jordan arms sale would make it even more difficult to line up votes for aid to the Nicaraguan rebels.