Good observation, but I think the phenomenon is not limited to the Left - and has a simple explanation if Left (and other) political writing is conceptualized as peddling intellectual commodity.
In most market situation, the competition takes place among individuals occupying the same market niche, not different niches. For example, the owner of a small coffee shop competes with owners of similar establishments in the area, not with grocery or hardware retail chains. Thus, that particular owner may be kvetching about a Wal-mart or a Home Despot behemoths being built in the neighborhood - but mainly because that is what her coffee sipping customers like to hear. But a Wal-mart or a Home Despot outlet does not really affect her business that much - and may even have a beneficial effect by attracting new potential customers to the area.
However, the owner of that coffe shop would be all up in her arms if a similar coffee shop were to open in the neighborhood - because that new shop would likely take away some of her customers. In fact many shopping centers, fully aware of that fact, have policies of not letting their premises to competing businesses. Not long ago, my community association put up a fight against a store owner applying for a liquor license, and we were strongly supported by the owner of a neighborhood liquor store in the area. Obviously, the owner did not like the prospect of having competition.
A similar logic applies to peddlers of intellectual commodity. Most US politics is in fact a platform for selling intellectual commodity - lectures, articles, books, pamphlets, signs or even popularity and name recognition (that can be turned into a better employment prospect), etc. Peddlers of different types intellectual commodities naturally compete with each other, and the smaller their market niche, the fiercer the competition. The market for mainstream intellectual commodity is quite large, so those who produce for that market can make a handsome profit even if their product reaches a relatively small share of that marker. But those who produce for small market niches cannot afford loosing their share to competition, because the numbers are so small. Hence they compete fiercely with each other rather than with the producers of commodities marked for mainstream audiences.
Needless to add that mudslinging always sells - so that too adds to the marketability of the intellectual product.
Wojtek