Democratic
- Working class urban Irish Catholics in Boston
- Hispanics in South Texas (for now…)
Republican
- Appalachian Unionists (especially Southeastern Kentucky and East Tennesse)
- The small town/rural/(and later) exurban upper crust outside of the South
Democratic
- Middle class urban voters in Manhattan with a liberal arts background
I’m not quite sure what precise definitions you’re using here (blame the fast-and-loose way Americans use the term “middle class” 
), but, either way, I very much doubt that this group has been anything close to reliably Democratic over this time period.
Assuming that by “liberal arts background” you mean that they have an undergraduate degree in the liberal arts, then for a very long time, university education was the preserve of the privileged, and thus the preserve of Republicans. In 1948, well over 70% of college graduates voted Republican, and this was even after the “liberal intellectual” phenomenon had certainly started to emerge. In addition, for much of this time period, having this sort of background in Manhattan would also likely mark you out as a WASP, which it goes without saying would strongly imply Republican politics.
I purposely said “background” to try to capture liberal arts “types” and not restrict it to college graduates for long-ago time periods. Even among graduates, I think the math can work out. What percent of college grads, even when being one was rare, were liberal arts majors? They’re certainly more liberal than college grads at large in every era, IMO. Additionally, ones living in Manhattan are going to be much more liberal than the national average. I could definitely see 70% of college grads leaning GOP nationally while this specific major in Manhattan leans left.
Take those numbers, and say that college grads voted GOP 70-30 while the country as a whole voted D 49-45 in 1948. That implies college grads were 2.5 times more Republican than the country as a whole, so even in Manhattan, which voted 51.5% Truman-32.75% Dewey you'd expect college grads to break for Dewey (I'd guess ~50% Dewey-30% Truman). Going back, I think college graduates in Manhattan voted GOP in every election from 1864-1932, probably narrowly voted for Roosevelt in his 1936 landslide, and then voted GOP again until 1960 or 1964. After 1964, they've been safely Democratic, but that's not a group with undying party loyalty.
