There is a fascinating article by Andrei S. Markovits about the history of the Left since 1945 in the current issue of Dissent Magazine. What I really appreciate about the piece is not only the historical perspective it offers, but also that the European experience is included.
He divides these last sixty years into four segments (begun respectively with the end of WWII, 1968, the elections of Reagan and Thatcher, and the fall of the Berlin Wall) and talks about the particular developments during each time segment.
The most important thread traces changes in the role that workers have played. The main story he tells involves factors that have contributed to the unraveling of the historical link between leftist politics and workers. He argues that the Left has never been successful since the late 19th century without its main constituency being workers.
The upshot is that the Left either has to figure out how to realign itself with a revitalized labor movement or find another constituency that makes as much sense as labor always made during most of the twentieth century. Either way, the demographics just don't point to the Left ever being viable with its center of gravity located in the middle class. A leftist movement that does not care about workers and/or the poor doesn't have a chance of accomplishing anything.
One of the subplots in this has to do with how the organizational strategies and cultural patterns of traditional unions have somewhat inhibited advances into new labor arenas: jobs traditionally held by women, unskilled labor, highly educated professions, and international organizing. The implication is that unless these gaps are more adequately dealt with, the whole leftist project is questionable.
The other interesting subplot is the emergence of two issues that have come to define the Leftist identity throughout the world far more than any other issues: anti-Americanism & anti-Zionism.