Arguably, the Civil Rights Movement did not really begin to pick up steam until 1955, after the murder of Emmett Till and the Montgomery Bus Boycott, led by Rosa Parks and Dr. King. However, it is true that some inroads were made in 1948 during the Truman Administration. The efforts of the NAACP at this time, which pushed for full equality, should also not be underestimated.
In 1948, Truman integrated the U.S. Armed Forces. He appointed a committee to study the problems that might ensue if blacks and whites were to serve together. The report was called "Freedom to Serve." The report outlined the steps that would be necessary to integrate the military.
In 1949, all jobs were opened to qualified personnel, regardless of skin color. Later that year, Truman issued an executive order requiring fair employment in federal service jobs. In public housing, too, efforts were made toward integration.
Predictably, Southerners were outraged. Their response was to form a Dixiecrat party -- that is, a Southern party committed to the rights of the common, working white man and segregation. Truman, nevertheless, remained firm in his position on integration. This antagonism from Southerners would foreshadow President Johnson's difficulties with Southern Democrats in the 1960s.
It is important to note that many black people, for many years in the 19th- and 20th-centuries, were Republicans. The Republican Party was the party of Lincoln and, thus, the party for liberty. It was not really until the election of Franklin Delano Roosevelt, Truman's predecessor and the man under whom he served as Vice President, that black people began to be courted by the Democratic Party.
The Election of 1960, famous for being the second-closest in American history, resulted in the inauguration of John F. Kennedy. In the first two years of his presidency, Kennedy mainly concerned himself with foreign affairs, particularly the threat of Communism. It was not until 1963, while watching the violent confrontation between police and black citizens in Birmingham, Alabama, that he realized that the matter of civil rights could not wait. Unfortunately, he could not turn his epiphany into legislation. Nevertheless, he and his brother, Attorney General Robert Kennedy, had frequent dialogues with major figures in the movement, and were viewed favorably by black constituents. This favorability, along with the off-putting conservatism of Republican candidate Barry Goldwater, allowed for Lyndon B. Johnson's landslide victory in 1964.
During his years in the Senate, Johnson pretended that he was in league with the Dixiecrats. However, once in office, and under pressure from Civil Rights leaders, he signed the Civil Rights Act of 1964. In a famous quote, he announced that, by signing the document, the Democratic Party would lose the South for good. He was right. Notice that, today, Southern states -- with the exceptions of Virginia, Maryland, and occasionally, North Carolina and Florida -- now prefer Republican presidential candidates.
The following year, Johnson signed the Voting Rights Act of 1965, which sought to address rampant voter disfranchisement in the South. The Voting Rights Act eliminated the poll tax and citizenship tests, common tactics used to keep black people from voting.
The Democratic Party remains the party that is most sympathetic to the concerns of the working-class. This is evident in its policies. However, many working-class whites, those who would have been Dixiecrats fifty years ago, now vote Republican. Ronald Reagan, a Democrat in his youth, once said, "I didn't leave the Democratic party. It left me." The sentiments of many white working-class and white middle-class people are echoed in that quote.
The Democratic Party has changed in that it no longer focuses solely on the concerns of working-class white men, in the style of Jacksonian Democracy, but tries to address to concerns of people of all backgrounds. It also seeks to be a progressive party, not only concerned with Civil Rights for blacks, but also for Latino immigrants and LGBT citizens.
So, yes, the Democrats have lost some white, working-class voters, but it has also gained many voters of color.