It's a shame that Democrats aren't passionate enough to have factions like this. But I suppose it's also testament to how polarized and extremist the individual stances within the Republican Party tend to be, like the "all taxes are theft" crowd, or the hyper religious crowd. Neither is particularly willing to compromise and get along with anyone. In contrast, Democrats seem to be much more able to compromise with each other. The progressive civil rights crowd, the environmentalist crowd, the pro social safety nets crowd... they manage to work together a lot better than different groups of Republicans do.
I think the right wing has too many positions that it's too stubborn about. But perhaps the left could stand to yell a little louder (or at least yell at all) about a few issues, to keep the party from complacency. I appreciate that this cohesion makes the left win more elections, but it also makes it drag in the mud a bit more.