A $421 million emergency spending bill is awaiting approval by the Senate, but the measure seems to be turning into a pawn in the election-year struggle over aid to the Nicaraguan Contra rebels. The Senate Appropriations Committee sent the supplemental spending bill for fiscal 1988 to the full Senate on Tuesday by a voice vote without objections. The Senate could vote on the measure as early as Wednesday, but maneuvering between Democrats and Republicans over Contra aid made the timetable unclear. Senate Minority Leader Bob Dole, R-Kan., wants to attach a $47 million Contra aid package to the supplemental spending measure. Democrats are trying to write a milder formula for Contra aid, which they want to insert into a Pentagon bill.