— First point of inflection: 1912 - a generation after Lincoln, Democrat William Jennings Bryan
campaigns for decades on an antitrust, populist platform. He fails, but Woodrow Wilson
carries on the attack and wins the Presidency, delivering a terrible blow to the pro-business Bourbon Democrats
. Meanwhile, an enormous fight breaks out in the Republican party, where the liberal Teddy Roosevelt
is so marginalized by Taft’s conservative faction that he creates his own party, fracturing the Republicans just as the Democrats tack left.
Second point of inflection: 1929 - yes, the Great Depression
happens. Democratic response through the New Deal
forms much of the basis of the modern party’s economic platform, though elements of the legislation are significantly racist
(the inclusion of which represents one of the last major compromises between the Northern and Southern Democrats). The Republican party, as the only major opposition to the Democrats, is a haven for the anti-interventionists and big-business types of all sorts who hadn’t already left with the Bourbon Dems.
Third point of inflection: 1948 through 1970 - by the 1948 DNC
, the pro-civil rights Democrats have gained enough influence to insert a civil rights plank in the party platform. Southerners are incensed, and the resulting split effectively creates two Democratic parties. The civil rights fight culminates in Lyndon Johnson’s
signing of the Civil Rights Act
in 1964 and the Fair Housing Act
in 1968, wherein he effectively asks white supremacists to get the hell out of his party. Meanwhile, sensing that things have come to a head on the other side of the fence, Richard Nixon
and the Republicans adopt a Southern Strategy
and welcome in the erstwhile Democratic racists of the South, creating the final lines of the parties we know today.
Basically, to understand how the Democrats became liberal (and the Republicans conservative), you have to understand 1912 and 1929. To understand how the Democrats stopped being racist you have to understand 1948 and 1968. I skipped a lot of detail, but there’s enough there to get from one end to the other.