The Democratic Party's support of the Civil Rights Act of 1964 primarily led to the withdrawal of many members in the South.
Historically, the Southern Democrats, who were part of the "Solid South," had been a key constituency for the Democratic Party. However, the party's commitment to civil rights and the passage of the Civil Rights Act alienated many white Southern voters. These members, who were often opposed to desegregation and civil rights for African Americans, began to leave the Democratic Party in favor of the Republican Party, which positioned itself as the party of the status quo and appealed to their interests.
This shift significantly altered the political landscape in the United States, contributing to the gradual realignment of party support in the South toward the Republican Party, particularly in the decades following the 1960s.