..."opinion blows back and forth".    It's not as simple as that.   

Politicians try to divine what their voters want to hear and tell them that.   Once they have done so and been elected, if their conduct differs from what voters were told in the election, groups monitoring them may well publicize this.

This applies especially when the politician was elected on the basis of opposing outrages perpetrated by the other side.   If he or she then were to compromise with the alleged evil forces (the other side), that politician may well be out on his or her ear at the next election. Particularly the case with House members--who have to run every 2 years.

Hence extreme partisans--especially in the House which was in fact set up expressly to be more directly responsive to the electorate than the Senate.   And very likely gridlock, if as now, the split is pretty even in the Senate, and closer than it was in the House.