Why did republican and democratic parties switch their views?

The Republicans were previously the champions of social, and to a much lesser extent economic, liberalism, following in the footsteps of Lincoln. The Democrats became more economically liberal during the Great Depression, leading to FDR's New Deal, but still had a strong support base among whites in the former Confederacy. But by the sixties both parties were fairly supportive of civil rights. When LBJ signed the Civil Rights act of '64, he famously said "There goes the South for a generation." In the election of '64, the South (and only the South, plus Arizona) voted Republican because the other option was LBJ again. In '68, it went to a third party. But the parties continued shifting. The Democrats had a run of weak candidates (and the Republicans had Reagan), and came out of it with a platform of basically "The Opposite of the Republicans," which created the modern political era, marked by the parties basically opposing whatever the other wants.