The Democrat Party did start out more for the people and strictly to the Constitution, and at this time they began to build their network for getting their message out, that was a network of newspapers (today they have added TVand Movies), but over time the Party began to stry from those roots and began standing with the rich and racist opressive policies.  what I see as one of their first racist acts come into play around 1846 with the expulsion of eastern American Indians.  Both major parties at the time were divided on the issue of slavery, another part of their network they began to be affiliated with was the labor unions.  Back to this later.
I will also come bak to slavery in 1848 the DNC was created and as is often the case even today the party is divivded the northern democrats were antislavery and the southern proslavery.  By 1854 the main Democratic leader in the Senate Stephen A Douglas pushed through  the pro-slavery Kansas-Nebbraska Act.  Many of the northern Democrats joined the newly formed Republican party that were anti slavery. The Republicans argued theat slaveholders had seized hold of the federal government and were blocking progress of liberty. 
As I read and learned more it looks like the party began attracting the lower classes but here is were things as I see it began to change for the party.  President Wilson ordered the segregation of the federal Civil Service, which tells me that desegrgation started in the early 1900's.  It was not until the 1924 Democratic National Convention that the party tried to denounce the KKK, although it was not directly tied to the party now it still ran with the party even today I think as evidenced by Senator Robert Byrd who fomrmed his own chapter of the KKK.  step forward to FDR who really altered the party away from capitalism toward economic regulation  and insurance against hardship or more plainly put government control and government hand outs that hold you down welfare.  He really began to grow the government with nationalized welfare, Social securty, and grreater regulation on business and transportation and communications. 
All this regulation and taxation did was make businesses have to raise their prices to follow the regulations and pay the taxes.  Now up to this point Blacks or the politically correct phrase African- Americans, voted with the Anti-slavery party the Republican party, but due to conditions of the time and the slow change in some areas that was holding blacks back but was changing, they saw what they Democrats were calling help as but was it really help? In the word of Southern Democrat Russell  "We will resist to the bitter end any measure or any movement which would have a tendency to bring about social equality and intermingling and amalgamation of the races in our (Southern) states." this he said against the Civil Rights Act of 1964.
I do not wish to overload you with information so this is it for now escept to note take caution on where you get your history as the history I got from the Democratic Party website was truly a washed  rosy devoid of any facts history.