Looks like the areas of the country with the strongest GOP trends in 1976 were New England and the West. Illinois, New York, Pennsylvania and Ohio (the four most populous states after California at that time) also trended GOP. There are two important things to remember about Carter in 1976 that I think explain these trends:
1) Having been a Southern governor, Carter's relationships with labor unions were not as good as most national Democrats. The South throughout almost all of its history has been staunchly anti-Union and the fact that Carter had not been visibly more pro-labor in state politics probably hurt him with blue collar workers in MI, PA, OH and other parts of the Industrial Midwest.
2) A lot of McGovern '72 voter might have not perceived Carter as being sufficiently liberal in 1976 (the whole Southern Baptist preacher deal and all) and might have stayed home, voted for McCarthy, or even pulled a lever for Ford (who had ended American involvement in Vietnam). There might be more of these voters in New England and parts of the West.
I think its hard for us to imagine in today's political environment how much of Carter's appeal in 1976 was regional. States like Arkansas and South Carolina were literally having 30 point swings to the Democrats from 1972 to 1976. That kind of immense regional appeal was necessarily counteracted with decreased enthusiasm for Carter in the North and West because it was perceived (by everybody) that Carter would be a Southern President.
To be fair, McGovern was reasonably liked in The West and with a more competent campaign in '72 against a challenger probably would have won those states all while The Northeast and South jointly said otherwise.
Carter was about as far away from McGovern as could be had for the time.
The better question though is how many Republicans crossed over for Carter just because he appeared more culturally conservative.
Only 9% of Republicans voted for Carter in 1976.
Also, he received 29% of the conservative vote (compared to 70% for Ford).
