All the Synthesis writers agree that one pervasive U.S. left problem is fragmentation, in which separate efforts dilute strength and compete for resources and status. Surely the gains in enlarged outreach, increased membership, and economies of scale are obvious. Each party, project, and movement has little time for what they see as spurious efforts at unity that won't advance their day to day survival and may even siphon energies from it. More, each worries that in unity its priorities will be subsumed by the scope of a larger ally or overrun by the energy of a smaller one. Large parties, projects, and movements bemoan the hassle of imbibing smaller efforts with their fanatical attention to things peripheral or distracting.
Smaller efforts fear to dilute or compromise their focused intentions and radical views.
Regardless of size, everyone fears to reduce their prioritization of race, class, gender, sex, war, ecology, or even sharper focus, or their special understanding and commitment and radicalism, by aligning with groups emphasizing something of lower priority, or insufficiently radical, or too extreme, or dead wrong. List the agendas and understandings of each potential ally and survey them for features in common you can work together around. Everyone in every involved party, project, or movement favors everything that is jointly sought, usually a pretty short list. The process involves little mutual learning. Each participant joins and later leaves, trying while in the coalition to benefit itself in context of a temporary intersection of a few priorities.