Historical note:

The U.S. Senate had approved the the Authorization for Use of Military Force Against Iraq Resolution by a vote of 52 to 47 on Jan. 12, 1991 (the majority including 42 Republicans and 10 mostly Southern Democrats). The House of Representatives approved the measure 250 to 183, with 164 Republicans and 86 Democrats in favor of the use of force, 179 Democrats, 3 Republicans and 1 independent opposed.

Neither house would have passed the authorization without some Democratic support since they had majorities in both houses. If they had been as monolithically against it as the Republicans were for it--Bush would have invaded anyway and there were not enough votes in the Senate to sustain an impeachment.