A lot of Northern liberals.
Yeah, this. Here's a 1956-->1984 swing map, using the standard Atlas key (every 5% extra, goes up 10% of shading); note that some of the >90% Southern swings "should" be much darker, but can't be. AK, HI, and DC are portrayed as zeroes in 1956, since they didn't vote.
None of MI, NH, or DE are errors.
Reagan's coalition
seems significantly better for only improving by 2 points nationally (from 58-42 to 59-41), but that might just be my vantage point in the 21st century; obviously in the 1950s the East was much more populated relative to the West, so weaker performances in CA and TX didn't matter as much, and the Midwest was more significant.
