Philadelphia voted Republican in the 1946 Gubernatorial and Senatorial elections, FWIW.
Republicans won all of the Philly Congressional seats in 1946 as well, though a number of them did flip back in 48.
The downfall of the Republican city machine and its replacement by as yet unbroken Democratic dominance. The Mayoralty flipped in 1952 by about 100,000 votes if memory serves me and the Democratic mayor Joseph Clark went on to become a Senator for two terms from 1957 until 1969. I think a big culprit is generational change and demographic shifts post New Deal finally caught up to the Republican machine and caused it to collapse. There was also internal corruption and other standard issues that tend to cripple machine operations in time.
the gop were still able to contest philly for a while after till the 80s with a dead cat bouce because of streets in the late 90s and early 2000s they elected da as well who was arlen specter who won the city in 1980.
