Black Americans were beneficiaries of Democratic policies, including President Truman's 1948 signing of Executive Order 9981, committing the U.S. government to integrating a long-segregated military.
Civil rights programs under President Kennedy and, more dramatically, under President Johnson (the Great Society program and his role in the passage of the 1965 Voting Rights Act) further cemented the relationship between blacks and Democrats.
But by then, the split that started between the Democrats and many of their white southern counterparts — who bristled at Truman's 1948 desegregation order and the Democrats' subsequent support of the civil rights movement — had widened into a chasm.
The Dixiecrats — those disaffected Democrats who bolted to the Republican party in 1964 — were pivotal to the success of Richard Nixon's “southern strategy” in 1968, and formed the nucleus of what would ultimately become the modern GOP.