Ever since 1932 the Dems have positioned themselves, with some justice, as the party that has more real help to offer the poor and unfortunate. A lot of those are to be found in big cities. Therefore Dems get elected to run the big cities. Which does not mean it is fair to blame the Dems for the socially dysfunctional results urban poverty also produces. Nor does it mean those results would fade away under a Pub municipal administration.