Why did the northeast swing so much between 1956 and 1960?

[Southern Senator North Carolina Yankee]

1960 emphasizes the decline in voting power for WASP Yankees as a force that could dictate the outcomes of the NE states (except for ME, NH and VT) and also the decline in the GOP performance with that group beginning to take hold. Though slight, the GOP under performed among this group relative to their 1948 numbers and remember 1948 was considered to be an under performance itself. Of course the bottom would fall out in 1964 for this group and though the GOP would recover somewhat in 1968, Nixon was down massively in for example the UES relative to 1960, which in turn was a week performance compared to 1948.

Republicans had made massive gains among middle class Irish/Italian/Germans during the 1950's under Ike (a process that really began to manifest in 1940) and in the grand scheme of things it was among these voters that Nixon would win in 1968, but 1960 was an reversion of the trends back to the old Catholic Democrat versus Protestant Republican one that had existed for a century, the difference between that being a winning coalition in the past and a losing one in 1960 is down to the relative vote shares in 1960. It should also be noted that some of these middle class Catholic precincts actually trended to Goldwater, though after 1960 that wouldn't be hard to accomplish.

Of great importance was the lesson that Republicans drew from both 1948 and 1960, as well as the disappointment with the African-American numbers in 1956. The inability to reconstruct the old school GOP coalition of Yankees, blacks and just enough of everyone else to win in a post New Deal era, is what helped encourage the GOP to seek out a newer more reliable base that could actually deliver results. This is essentially the case that Kevin Phillips made, paraphrasing of course: "The NE won't deliver, not even Rocky can win a majority of the black vote, focus on the South and the West and let the Democrats have the old pastures". Also, "any subsequent success in places like New York will be driven by Irish/Italians/Germans, voting in opposition to the interests of WASP elitists (think Staten Island's voting behavior), not the other way around".