I've been reading about the 1920 election and am struck by how hard the cities swung against Democrats and the degree to which the Harding campaign successfully managed to court recent immigrant groups. It seems that even the Irish who were the most reliable Democratic voting bloc in the north may have voted for Harding by a plurality. Sometimes I wonder if the Democrats were on track to be reduced to a southern regional party with the GOP becoming dominant in big cities were it not for the Al Smith campaign and the New Deal more then reversing those trends.
That's very possible, but I think the Al Smith campaign had at least as much to do with it as the New Deal, when adjusting for the total, sizable impact of both things. In other words, Democrats were already "the party of immigrants" (and, in general, always the party of "the marginalized," at least as they chose to define it at given periods) well before Al Smith, and Smith's campaign seemed like a very intentional effort to get back to the party's roots in that regard. The New Deal provided them with a very logical next step on that path, IMO.