Critics and defenders of the Reagan administration's request for aid to Nicaraguan rebels began staking out battle positions yesterday, led by bipartisan group of 18 senators who asked that the package be withdrawn.

"This would be a particularly bad time for the United States to increase the level of conflict in Central America," said the senators, 16 Democrats and two Republicans. They asked that the request wait until President Reagan holds further discussion with Latin American leaders engaged in regional peace negotiations.

The president's request would provide $30 million in nonmilitary aid to the contras, or counterrevolutionaries, and give him $70 million to use in any way he deems appropriate, presumably for military assistance. House leaders met to discuss ways of handling the request.

In a move that one administration official said was related to the contra aid package, State Department officials finished assembling a five-year, $250 million aid request for Northern Ireland and are expected to submit it to Congress in the next few days.

That aid, outlined in general form in November, would come in the form of a U.S. contribution to an international fund to help economic and social development along the border between Northern Ireland and the Republic of Ireland.

The aid request is expected to have broad support on Capitol Hill. Its chief supporter is House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), a leading critic of contra aid. One House aide said, "I suspect an attempt may be made to attach contra legislation to the Irish aid bill or vice versa" in an effort to make the contra bill more attractive or to get it to the floor unchanged.

An O'Neill spokesman denied any link in the speaker's mind between the requests. "The speaker is the greatest supporter of Irish aid and the biggest opponent of aid to the contras," he said.

Earlier, GOP legislators told Reagan that, despite their backing, the contra aid proposal faces rough going on their side of the aisle.

"We have a great deal of missionary work to do," House Minority Leader Robert H. Michel (R-Ill.) said after the meeting. But Michel added that he is "not ready to do any compromising."

Reagan met with 22 House members yesterday morning and 22 senators in the afternoon, along with Vice President Bush, the secretaries of state and defense, his national security affairs adviser and the director of the Central Intelligence Agency.

"The administration and those of us who have supported them in the past need to do a better job of explaining what the administration's goals are," said Sen. John Heinz (R-Pa.). "Without people being clear about what the president's goals are, I think that his package will run into difficulty."

Sen. Mark O. Hatfield, chairman of the Senate Appropriations Committee, said he opposes the aid request. "For every contra we supply with a new gun, countless elderly Americans will go without meals . . . . For every bomb we provide the contras to destroy Nicaraguan farms, another American farm will face bankruptcy," he said.

The administration wants an up-or-down vote on its contra aid proposal, but the request has been referred to four House committees, each of which is likely to want modifications.

Last year's legislation, which provided $27 million in nonmilitary aid to the contras and which expires March 31, set up expedited procedures for this year's measure. They mandate a vote on the president's request, as presented, within 23 legislative days, which could come as late as mid-April.

But the procedures also allow the House Rules Committee to assert jurisdiction over the measure. "You get in there and anything can happen," one Democratic aide said.