Illinois Republican Party congressmen were in turmoil today over a federally imposed reapportionment map that would strengthen the city's Democratic stronghold in Congress and eliminate at least two suburban GOP incumbents.

Stunned by a 2-to-1 decision by a panel of federal judges, party leaders were trying to decide whether they have sufficient legal grounds to appeal the ruling to the Supreme Court.

"This was an unusually bitter pill for Republicans," said Rep. Edward Derwinski, a senior member of the state's GOP delegation and one of the suburbanites put in a squeeze. "All 14 Republican congressmen want to appeal and now it's up to the lawyers to see if we have a case."

The federal panel accepted a map that had been drafted by Illinois House Minority Leader Michael J. Madigan, chief of Democratic Mayor Jane Byrne's forces in the lower chamber of the General Assembly. The panel rejected a proposal that would have eliminated two Chicago based districts and a compromise that would have sacrificed a Chicago Democratic incumbent and a suburban Republican.

The majority opinion of U.S. appellate court Judge Robert Sprecher and U.S. District Judge Susan Getzendanner argued that the Madigan map is the most likely of the three proposals to keep three blacks in Congress. The majority also said that the map recognizes Hispanic communities, properly reflects Illinois as a "swing" state, and most closely follows the one-man one-vote principle.

Dissenting Judge Frank J. McGarr, chief of the northern district of Illinois, argued that none of the three plans should have been adopted without changes. Two of the judges on the panel were appointed by Republicans and the third by a Democrat.

Illinois' current 24-member delegation will be reduced by two seats because of population loss shown in the 1980 census. Under the map adopted by the panel, the delegation, now 14 to 10 Republican, likely would be split 11 to 11 between the two major parties.

The map extended the Chicago-based district of Democratic Rep. Sidney R. Yates along the shore of Lake Michigan into north suburban Evanston, taking in the home of Republican Rep. John E. Porter. It also crowded into two districts three suburban Republican incumbents--Derwinski, George O'Brien and John N. Erlenborn.

Even though Chicago's population has been reduced over the last decade, all eight Chicago-based districts were maintained by reaching into suburban territory.

Porter announced that if the new map isn't changed he will move north into an adjoining district to challenge fellow Republican Robert McClory. McClory, who represents the northeastern section of the state, said he plans to run for an 11th term in "this territory that is familiar to me."

"We're going to huddle," Derwinski said of the other GOP incumbents. "We want to decide what to do before we start making comments against each other."

Two other Republican incumbents have ended up in swing districts, Derwinski said. They are Rep. Paul Findley and Rep. Daniel Crane from downstate Illinois.