The real answer is 1932. The New Deal coalition meant that the NE would default to the Democrats with the big city machines and unions, followed by the strong African-American support that started in 1936.
Republicans could win when turnout dropped and Democrats were unpopular like 1946 or when they had a popular General as President like 1952. But when they pissed the unions off, or the economy went bad, it was brutal (1948, 1958 and 1974). The Republicans couldn't win and sustain themselves on level high enough to get the majorities in Congress while being excluded from two regions. So they had to either deal and move to reflect the new reality, or look to craft a Conservative coalition in a different region. In so doing that over time alienated the populations that were part of their original, natural but now outvoted GOP base. However, it should be noted that said group was changing as well.
Religion decline massively amongst Protestants in the North and at the same time Environmentalism became really big in those same rural areas like Vermont. So the natural population of WASPs is now moving more and more left, and even more than that you had a massive influx of people like Sanders. Vermont would vote more like Maine if it were not for those migrations of left wingers, just like New Hampshire would vote more like Maine were it not for the in-migration of those Conservatives during the same time period.
The decline of religion at the same time the GOP was re-ambracing it caused the problem, not embracing it out of the blue alienating a historically secular base. The GOP had long been tied to Protestant zealotry in many areas. Second of all, for those increasingly secular voters who were lower middle and working class, the embrace of the sunbelt economic policies, removed those policies that could keep them on economic issues. So once again the native populations in places like those along the upper Mississippi and rural New England meant that neither wedge issues, nor economic ones provided any pull for those voters to vote Republican.
Finally, you have the death knell for the GOP in the suburbs of that region in 1992 once the recession hammered white collar people, people who thought themselves secured from such, and that it happened during a Republican Presidency, delegitimized them on the kitchen table issues at the same time that now not only Protestant, but Catholic religiousity was slipping and social issues would prove alienating. I agree, NJ was lost as a competitive state in the 1990's.
1974-Thats was because of "Watergate".
That's true the Northeast Republicans are more moderate on economic issues than their Sunbelt counterparts.
I don't think declining Catholic Religious Affilation hurt the GOP in "The North" but as you said keying the social issues rhetoric hurt the GOP.