Doubt it was election fraud. As liberal as Bloomberg was, he still had the R next to his name.
People voted for the D, not the name De Blasio. I worked in Manhattan for about 10 years and am VERY glad I no longer work in NYC. Bad days are just around the corner.
He is on the leading edge of a larger phenomenon as the Dem Party continues to move leftward. It will shortly drop all pretense and openly run as a Euro-style Social Democratic party.