I mean Carter was even leading even in early October according to wikipedia past election polls , so did that many people really make up their minds in the last minute . Even the last poll shows reagan only winning by 3 points instead of the 10 points he won by. I mean even this year the polls were not as wrong as that .
1980 is kind of a perfect storm, in terms of the results. I remember reading somewhere that Carter's pollster calculated a worst-case scenario in 1978/1979 that resulted in Carter carrying only
eight states- Georgia, Minnesota, Hawaii, West Virginia, Massachusetts, Rhode Island, Maryland, and Arkansas (DC too, of course). That was laughed off but it ended up being even worse... Carter shipped Cuban refugees into Arkansas and they rioted (or something- I'm hazy on this?), and that cost him an unwinnable state, while John Anderson stole a bunch of environmentally conscious yuppies' votes which cost him unwinnable Massachusetts. Sometimes a loser is a loser.
BUT: here's the results if you apply, say, a uniform six-point swing toward Carter:
So Reagan wins 341-197, 48-44 in the popular vote. I don't think many people at the time would have been surprised by
that result (except for Maine and Vermont, maybe- note they both flip in this scenario!). If Carter was just a little less inept or unlucky... if the Ayatollah released the hostages before Election Day... if if if.
EDIT: I can't post the map but Carter would have won the South (minus Texas, Florida, Virginia) and added Wisconsin/Delaware/New York/Massachusetts/Vermont (!)/Maine including both congressional districts (!!)