One minor caveat is that the Republicans did pretty well in the Northeast in the 1986 mid-terms. Their only Senate seat lost was Maryland, if you count Maryland as being part of the Northeast. The Northeast did well out of the Reagan boom/ bubble, and Reagan ran up large vote percentages in those states that seem weird compared to Republican performance in other late 20th century elections. The Republicans lost their Senate majority that year in the South and in the Midwest.
The Republicans didn't have that many northeastern Senate seats up for re-election in 1986: just MD, NH, NY, and PA. With just four seats at stake, I don't think much significance can be attached to their "losing only one." I think in each case, personal/local factors were decisive: Rudman and Specter were popular moderates, D'Amato was famous for his attention to New York problems ("Senator Pothole"). And the loss in MD was simply a matter of Mathias retiring (rather than face primary opposition from the right) and the GOP nominating a candidate too conservative for a state where Reagan-Mondale had been much closer than in the nation as a whole.
Buckley never managed to increase his strength beyond a conservative core in New York, and I'm not sure he could do so in CT. (Which is one reason it would be unlikely for him to get elected in CT in 1980 in the first place--unlike NY in 1970, it isn't a three-way race.)