Independents and weak partisans are more supportive of members that espouse a bipartisan image, whereas strong partisans are less supportive. People with strong attachments to a political party disavow conflict in the aggregate but approve of individual members behaving in a partisan manner.which is interesting right now: Barack Obama almost endorsed the Simpson-Bowles plan for the budget, which is a centre-right proposal, while Paul Ryan went to the extreme. The main graph is here:
Interestingly, this would tend to mean that partisanship is kind of self-fulfilling. If we think that a partisan congress polarizes the electorate, this means that the fraction of strong partisans, and this implies that congressmen have an incentive to be more partisan if they want to get reelected. We are in a typical story of "strategic complements" with multiple equilibria, one with strong partisanship and one with weak partisanship.