Marching With StalinistsBy Michael Kelly The left in America has for a long time now resembled not so much a political movement as a contest to see how many schismatics could dance on the head of a pin, a conversation that has gone from being national to factional to simply eccentric. At some point, progressive politics reached a state where freeing Mumia was considered critical and electing a Democratic president was considered optional. Then came Sept. 11, and the left found itself plunged into a debate on a subject of fundamental importance. And this was a debate in which to...