Answer:
The democratic party became the party of civil rights because Lyndon B. Johnson, the president who signed the 1964 Civil Rights Act into law, was a democrat.
Step-by-step explanation:
Southern Democrats opposed civil rights legislation, for example, southern democrat Strom Thurmond from South Carolina gave a 24 hour speech to try to fillibuster the bill in the Senate.
After the bill became law, most southern democrats switched to the Republican Party, as well as the electoral base. Partisanship was not as strong druing the 1960s, 1970s, or 1980s, but in recent decade it has become stronger, making the Southern United States a bastion of the Republican Party.