The South in 1980?

[sg0508]

Carter really didn't have anywhere to build from 1976.  The west and southwest were a lost cause.  He had to hold the Midwest and the industrial states in the northeast to win again.  Personally, I just think people stayed home, and Carter really seemed to have no base in '80.  Even the jewish vote (a very small percentage of the total electorate) gave Carter a terrible performance.

[sg0508]

It's striking how that last debate just completely blew the race from being perceived as "neck and neck" into a total electoral landslide.  While I wasn't alive in '80, my parents and grandparents had told me (in addition to reading up on the race) that prior to that final debate, it was still 50/50 as to who was going to win and many thought Carter still had a chance.

That last debate clearly cost Carter most of those southern states and big states like NY.  Also keep in mind that the challenger typically gets most of the remaining undecided voters in the final days, which Reagan clearly did.  That ironically, is what likely stopped Pres. Ford from winning in '76 and completing a remarkable rally after being down 33 pts in the popular polls following the DNC.

Again though, I firmly believe that many Carter supporters just didn't bother turning out and that's why those states broke away from him. 

[sg0508]

Also keep in mind that the challenger typically gets most of the remaining undecided voters in the final days, which Reagan clearly did.

Could you provide some evidence for this assertion?

Here.  https://en.wikipedia.org/wiki/Historical_polling_for_U.S._Presidential_elections#United_States_presidential_election.2C_1976