Why did Carter do so well in ME,VT?

[Bidenworth2020]

He almost won them in both 1976 and 1980

[Mr. Smith]

Muskie opened up the floodgates in Maine in '68.

[Sumner 1868]

Carter actually didn't come close in Vermont, actually, and failed to carry even fairly Democratic Chittenden County. He did, however, preform better in VT than all previous Democratic candidates except FDR and LBJ, which in hindsight is probably an early sign of the state's later shift.

As for the close result in Maine, the state's Republican machine had already collapsed by this point. 1974 saw Democrats take the State House and all four elected state offices. Ed Muskie  running for a forth term probably had an upticket effect as well.

[Fuzzybigfoot]

The 1982 Almanac of American Politics said many in Vermont felt alienated from Reagan due to his perceived conservatism on cultural issues.  It's good to keep in mind that Carter still did almost six points worse in Vermont compared to 1976 (he dropped from 43% to 38%), with Anderson carrying 15% in 1980.