The attractive outsider figured out the new primary system better than many old Pols. He put great effort into the Iowa caucuses and won 9 of the first 10 primaries. He was quite a contrast to Nixon and the coalition of Southern Democrats and Northern republicans who had ruled Congress- ""I’ll never lie to you, I am an engineer, not a wheeler-dealer". There was a small racial factor too. We saw the white Southern backlash that Nixon and Wallace had nurtured. Here was a racially progressive Southern governor. Yes, I mean the man from Plains, GA.
Consider this table listing Democrats in the House & Senate from 1974-1980
Composition of Congress 1972-1980
Democratic control peaked in 1976 and declined only slightly in 1978. The gas lines and the image of an ineffective dour President changed that.