Concerning Texas and the South (can't speak for California). I'll attempt a brief summary of many events, some of them bloody. It was a drastic switch and a drastic time.
The Democratic party began to lose the south with the passage of the Civil Rights Act of 1964 and the Voting Rights Act of 1965. (50+ years ago) This was the beginning of the move to the Republicans.
The 1960's had experienced major race riots in several cities, people were killed, National Guard deployed in the streets and property burned to the ground. The Democratic party was divided over civil rights and opposition to the Vietnam War. Conservatives supported the Vietnam War and liberals wanted to withdraw.
Lyndon B. Johnson (LBJ) was president and wished to enacted major legislation he called "The Great Society". Conservatives opposed LBJ's legislation and as the Democratic party became more liberal conservatives moved to the Republican party. LBJ continued to press forward with the war contributing to a divided Democratic party. (LBJ and Vietnam too complex for this narrative.)
The 1968 Democratic National Convention in Chicago saw riots in the street and heated debates on the convention floor. Conservative Democratics saw this as an erosion of law and order and felt the Democratic party was weak on law and order.
1968 was a very tumultuous year. In April 1968, Martin Luther King Jr. was shot dead. In June 1968, Robert F. Kennedy (RFK) was shot dead. Kennedy was running for president and seeking the Democratic party nomination. Kennedy was viewed as a liberal and conservative Democrats opposed Kennedy. RFK was a brother of John Kennedy, president and was assassinated in Dallas, TX in 1963.
Richard M. Nixon, Republican, was elected president in 1968.
With the Voting Rights Act, the percentage of African Americans registered to vote in Mississippi increased from less than 10% to over 60%. Mississippi was the most dramatic state increase but all the southern states saw increased registration by African Americans. (I don't have numbers for Texas.) White southern conservative Democrats supported the Vietnam War and disliked the civil rights acts and they switched to Republican.
Lyndon B. Johnson (LBJ), president and a Democrat from Texas, was instrumental in turning Texas Republican. LBJ pushed civil rights laws thru Congress knowing the Democratic party lost the southern vote perhaps for a generation or longer.
In 1964 LBJ had been elected with 486 electoral votes. Barry Goldwater, Republican senator from Arizona, ran against LBJ and received 52 electoral votes. Voters liked LBJ.
In four years LBJ became so unpopular he didn't run for a second term and in a nationally televised speech famously said, "I shall not seek and I will not accept the nomination of my party as your President".
http://scholar.library.miami.edu/sixties/urbanRiots.php
https://www.rollcall.com/news/party-divided-party-defeated-usually
article/we-have-lost-the-south-for-a-generation-what-lyndon-johnson-said-or-would-have-said-if-only-he-had-said-it/
https://archive.nytimes.com/www.nytimes.com/learning/general/onthisday/big/0331.html
Hi there - a reminder to those interested in answering this question that the 20-year-rule still applies, and that 21st Century political events are still out of bounds for discussion - but there is still much you can talk about in terms of demographics and political trends from before 1999.