Yes, this is the swing map:
Well the first thing that tells you is just how much of the legwork of flipping Massachusetts from Republican to Democrat was done by Al Smith in 1928. But if you go back further than that, Democrats were actually doing decently there in presidential election between 1876-1892. They broke 40% in each election other than 1880, and they only barely fell short there. In 1892, Cleveland managed to keep his loss in the state down to less than a 7 point margin.
Then between 1896-1924, Democrats bomb horribly in each election in the state other than 1916 where Wilson only lost the state by 4 points. Yes, Wilson narrowly won in it 1912, but only because Roosevelt and Taft split the Republican vote just enough for him to to squeak through, his performance on its own merits wasn't any more impressive than the past several elections had gone in the state for Democrats. Now, sure, 1896-1930 was the fourth party period and Republicans were dominant in the North during this period, but Massachusetts was absolutely awash with Irish immigrants and their descendants at this point, you would think you could at least detect a gradual weakening of the Yankee grip on political power in the state during this period, the shifting tide of the "turf war" as you put it. But the only hint you get of that is 1916, and based on how the two elections that proceeded it went, it would have been very easy to write that election off as a fluke pre-1928. It consistently voted to the right of the national average (except 1904 where it voted almost exactly in line with the national average). Then came 1928 and the Democratic vote totals increase more than 500,000 from what they got in 1924.
How did the Republicans manage to hold such an iron-fisted grip on both the statewide and federal politics of the state up until 1928, given the massive Catholic immigrant and descendant population residing there? Why did it take until 1928 for the dam to finally burst in an overwhelming fashion with very few "leaks" to speak of beforehand?
