Nate Silver explains:
When the truth behind a claim is hard to discern, voters sometimes use the heuristic of looking at what partisan actors say. If one side makes one argument, and the other side rebuts it word for word, the public may come to regard it as a typical partisan squabble or conclude that the truth boils down to a matter of opinion.
However, if one side makes its point unambiguously, while the other side hedges and does not seem to have its story straight, the public may conclude that the truth lies on the side of the group that has articulated its case more vigorously. This dynamic may have worked to the Democrats’ disadvantage during the health care debates of 2009 and 2010. While Republicans were nearly unanimous in opposing the the Democrats’ bill, Democrats were often fighting with themselves about its goals and its substance. Initially, more people supported the bill than opposed it. But as the debate unfolded, nearly all the voters who were on the fence came to take a negative view, and the bill proved to be quite unpopular overall. Whatever the weight of the objective evidence for and against the bill, the arguments the public heard were disproportionately against it, and they reacted as you might expect.