You are conveniently forgetting that John Anderson was also in the race and when he crapped out most of his support went to Reagan.
Once Anderson declared he was running he siphoned about 20% of Carter's votes and Carter remained in the low 40% range the rest of the way.
Once Anderson declared he was running he siphoned about 20% of Carter's votes and Carter remained in the low 40% range the rest of the way.