The term Reagan Democrat is used (with caution) by psephologists and (more freely) by political commentators to describe traditionally Democratic voters, especially white working-class ones, who defected their party to support Ronald Reagan, either in the 1980 election, or, more commonly, the 1984 one. Reagan Democrats were in some ways a culmination of a phenomenon in American politics whereby, from a date sometime between 1965 and 1984, a significant portion of the electorate which had traditionally supported the Democrats defected to the Republicans, at least in Presidential Elections.
This process is held to account for the size of the majority of the popular vote enjoyed by Ronald Reagan in the 1984 Presidential Election. Possible causes advanced for the shift in voting patterns include a backlash against desegregation in the southern states, a decrease in unionized workers, and the waning influence of the working class within the Democratic Party. Reagan was himself once a Democrat, but made the switch to Republican before the 1964 elections.
The classic study of Reagan Democrats is probably Stan Greenberg's 1985 work analyzing white voters, largely unionized auto workers, in suburban Macomb County, Michigan. He concluded that they no longer saw Democrats as champions of the middle class, but instead identified them as being the party of African Americans and the very poor.