I'll try posting in segments. Part 1:

From the end of Reconstruction in 1877 until the passage of the Civil Rights Act in 1964, the Democratic Party so dominated Southern politics that the former Confederacy was known as the Solid South, a consistent blue voting bloc. This ultimately changed because of the Democratic Party's gradual support for Black civil rights. From 1964 until 1994, the South slowly migrated from being a Democratic stronghold to being a Republican stronghold without drastically changing its political views. Social progressivism in the Democratic Party drove away its conservatives, who were then recruited into the Republican Party.

The Democrats and Republicans were once less ideologically homogeneous than they seem now. There were conservative Democrats, particularly in the South, and there were liberal Republicans. President Franklin Roosevelt built a New Deal coalition to support his programs to fight the Great Depression in the 1930s, blending labor unions, rural Southerners, and racial and religious minorities into a broad base of support for his agenda. The Roosevelt administration even recruited liberal Republicans into the coalition; the administration's efforts to include minorities in the coalition led to the Black Cabinet, a group of Black leaders that served as public policy advisors to the president. The New Deal coalition was opposed by a conservative coalition, primarily composed of conservative Republicans and conservative Democrats, the latter of which was predominantly made up of Southern Democrats. These two coalitions would outlive Roosevelt and define American politics until the 1960s, and served as the axis around which Southern Democrats would migrate to the GOP. The result of these coalitions was an increasingly progressive Democratic establishment at odds with a large conservative voter bloc in the South over issues such as social welfare and racial integration.

In February 1948, President Harry Truman desegregated the US armed forces. He ran for re-election with a moderate civil rights platform that angered Southern conservatives and led to a break with the Democratic Party. Dissident Southern Democrats, now calling themselves Dixiecrats, ran a third party ticket as the States Rights Democratic Party under South Carolina governor Strom Thurmond, who was a Democrat at the time. In the first major weakening of the Solid South, Thurmond received 1 million votes and 39 electoral votes, winning Louisiana, Mississippi, Alabama, and South Carolina outright. Of course, Truman won the election of 1948 despite the mutiny of the Deep South, and Southern conservatives continued to support segregationist Democrats. Notably, the GOP began to chip away at the Solid South during Dwight Eisenhower's presidential elections in 1952 and 1956.

Southern Democrats led the segregationist movement against Black civil rights during this era. Virginia Democrat Harry Byrd was the architect of the "massive resistance" campaign against school desegregation, and 99 Southern Democrats signed the Declaration of Constitutional Principles (also known as the Southern Manifesto) in opposition to desegregation efforts. Strom Thurmond's individual filibuster against the 1957 Civil Rights Act is still the longest in history, at 24 hours. Southern governors like George Wallace in Alabama and Ross Barnett in Mississippi were among the faces of the segregationist resistance. When John F. Kennedy arranged for Dr. Martin Luther King to be released from prison after MLK was arrested for participating in a sit-in in Atlanta in 1960, there was another Dixiecrat mutiny. 14 electors from Southern states won by JFK in the 1960 presidential election refused to cast their states' votes for him, instead casting them for Harry Byrd despite the fact that he was not a candidate in 1960.