I think liberals massively overestimate the degree to which poor messaging contributed to their difficulties. That isn't to say, though, that Democrats don't have a problem here. NBC's First Read:
Want another example of how Republicans play political hardball better than Democrats do? Just look what happened after yesterday’s House vote extending only middle-class tax cuts. We noticed only a few Democratic press releases accusing Republicans of voting against tax cuts for 98% of Americans (and thus accusing them of raising these folks’ taxes). If the shoe had been on the other foot, however, Republicans would have mercilessly pounded Democrats for weeks -- if not months. The recent Charlie Rangel and John Ensign stories are instructive here, too. Republicans were relentless that the news of Rangel’s wrongdoings never got dropped. On the other hand, Democrats essentially gave up on Ensign’s woes. And lo and behold, it now looks like Ensign is no longer a Justice Department target. And Rangel's been censured on the House floor. Republicans just play the political message game better than Democrats do.