Brythan's answer discusses the shift of black voters from the Republican Party to the Democratic Party.
In the past half-century, social issues have driven large numbers of married people (especially those who oppose abortion) from the Democratic Party to the Republican Party. As explained below, these people are disproportionately White. Four major social factors have been:
- a racially-correlated timing factor in the Sexual Revolution
- Johnson's "Great Society" programs
- the Democratic Party's support for abortion, and
- the religious demographics of the South.
1) Broken home rates among American Blacks were high even before the Sexual Revolution. In the early 1960s (when Moynihan brought political attention to the issue) they were in the 20 - 30 percent range. In the wake of the Sexual Revolution, they rose to the 70 - 80 percent range for blacks, and the 20 - 30 percent range for whites.
2) The "Great Society" programs provided direct financial assistance to "impoverished" people. This created high marginal tax rates for the poor. Broken homes cause poverty -- and some of the "Great Society" programs encouraged broken homes. Because Black broken home rates have been higher than White broken home rates, discussions of the problems of the "Great Society" took on racial connotations. The Democratic Party has positioned itself as promoting, defending, and expanding the "Great Society" programs.
3) In the late 1960s and early 1970s, most U.S. states loosened restrictions on abortion. In 1973, the U.S. Supreme Court ruled that all of the U.S. states had unconstitutional restrictions on abortion -- despite the U.S. Constitution being silent on the subject. There nearly was a constitutional amendment to overrule the Supreme Court. This amendment was narrowly defeated by Democratic party politicians who said they were "personally against abortion", but consistently voted to legalize and/or publicly fund abortion. Since then, it has become increasingly difficult for anti-abortion Democrats to be elected as Democrats; whites who are "personally against abortion" have increasingly switched from the Democratic party to the Republican Party; and it has become increasingly difficult for pro-abortion Republicans to be elected as Republicans. Blacks (perhaps because of their higher broken-home rates) have much higher abortion rates than Whites.
4) A disproportionate number of Southern whites are members of Fundamentalist Christian churches. These churches have organized their members to oppose abortion.
Notable Republican operatives (including Lee Atwater and Karl Rove) have noted these trends, and deliberately used social-policy "wedge issues" to break up the post-LBJ Democratic Party.
Steve Sailer has analyzed the "Marriage Gap" and "Affordable Family Formation".