The Democratic Party's support of the Civil Rights Act of 1964 primarily led to the withdrawal of many members in the South.
The Civil Rights Act aimed to end segregation and discrimination based on race, which was met with significant opposition from many Southern Democrats who were aligned with the segregationist policies prevalent in the region at that time. As a result, many white Southern voters and politicians left the Democratic Party, leading to a shift in the political landscape in the South and contributing to the rise of the Republican Party in those areas. This realignment was part of a larger trend where the Democratic Party became increasingly associated with civil rights and social justice, while the Republican Party began to attract disaffected white Southern voters.