I recommend two related articles. First, Jonathan Chait’s article Why the Democrats Can't Govern in The New Republic. Then Sean Trende’s related article, The Real Reason The Democrats Can't Govern on the Real Clear Politics site.
Despite the titles, neither of these is a right-wing polemic. They are instead an analysis of how structural factors in our Constitution, and in the way we elect representatives, makes it harder to build effective left-wing coalitions than right-wing coalitions. Well worth reading.