Following groups Reagan did pretty good with.
White Nationalistic and/or Socially conservative registered dems. Regardles of whether they were evangelicals, more religious or working class social conservatives who never bothered to go to Church they liked him. These guys probably fell out with the democrats and voted for Nixon or became Reagan supporters but never bothered to change their registration to Republican until then. In the following years this group finally changed their registration which is why the democrats who used to have a high voter registration advantage(not that it helped them as much as they hoped) the gap in registration became lower and lower.
White Suburban registered dems, many of which also had the same ideas as the first group. Many of these were young when they registered as democrats and once hey got degrees and good paying jobs they wanted to pay less taxes.
One thing to note is that democrats were super good at registering voters and they thought that made them stronger in elections if they registered voters when they were young. However many people who registered as a democrat when they were under-35. Ended up being convinced to vote GOP in the following years mainly because of religious, or social or nationalistic or etc issues/positions. Thats why states like Oklahoma and Louisiana and Kentucky for a long time had a large democratic voter registration advantage that ended up not being that useful for the dems because a large segment decided to vote republican.
Things were shltty, and Reagan gave a compelling case for why he would make them better in a much less partisan time. For many Americans, by 1984, he had.
Basically this. After stagflation, Vietnam, the unrest at the end of the 60s, Watergate, and the weak Carter administration, Reagan made America believe in itself again. Also in 1984 the economy was booming for the first time in many years.
Job growth during Carter's 1st term was 10 million. Both of Reagan's terms resulted in 18 million. And this was despite the high inflation from the Iranian revolution.
