As a whole, the Dems are pretty center of the aisle, because America as a whole is fairly conservative compared to Europe (despite 60% of the population being more liberal than the government at most times). Europeans generally consider the Dems in the US a conservative party, and corporate Dems are definitely closer to the right than to the left. The other issue besides the general conservative leaning in the country though is that there's about 50 other groups of various left leaning shades that would be their own separate parties in Europe but are bunched in with the corporate Dems and therefore have little say in the party platform.