Supporters of the Democratic Party are often viewed as educated middle-class Americans, often white collar, but not particularly wealthy (read, not in the top 2% financially) along with urban blue-collar workers - those that are concerned with positive social programs, essential freedoms, and equality for all. Supporters of the Republican Party are often thought of as one of two types of people: wealthy fat cats that use their wealth for personal gain, and lesser-educated people, generally poor and white (but not always so), living in rural areas, who are often termed rednecks and/or racists (I know this is far from true, but this is the common stereotype - Republicans have George W. Bush to thank for that).
Let's set the way back machine for 1860, when Abraham Lincoln, the first Republican President, won the nomination and eventually the election for President. Three years later in 1863, he abolished slavery with the Emancipation Proclamation, which led to the passage of the Thirteenth Amendment to the Constitution of the United States in 1865, which abolished slavery on a Constitutional level. Lincoln won two terms in office, and energized the Northern States so much that, along with Lincoln's reelection in 1864, every Northern state also elected a Republican Governor, and both the Senate and House also had overwhelming Republican majorities. Lincoln was assassinated on April 14, 1865 (shortly after the Thirteenth Amendment had passed into law) by John Wilkes Booth, a man who sided heavilly with the South and Southern politics, and who was likely to be a member of the Democratic Party, though I can find no information to back up my speculation on his party affiliation.
Meanwhile, the Democratic Party, founded in the early 1790s, had been split down the middle because of the Civil War, resulting in the Northern-based War Democrats and the Southern-based Peace Democrats. Much of the War Democrats party sided with Lincoln, and became Republicans. The War Democrats party faded into oblivion shortly thereafter, leaving the Southern-based Peace Democrats, who ultimately became just the Democratic Party.
As we are all taught in grade school American History classes, the South was not happy about their defeat in the Civil War and the abolition of slavery. This resentment of progressive politics and a declaration of freedom for all, regardless of race, gave rise to the Klu Klux Klan. The KKK, being mostly Southerners, who were mostly Peace Democrats, were the most supportive of the Democratic Party. In fact, historian Eric Foner observed:
In effect, the Klan was a military force serving the interests of the Democratic party, the planter class, and all those who desired restoration of white supremacy. Its purposes were political, but political in the broadest sense, for it sought to affect power relations, both public and private, throughout Southern society. It aimed to reverse the interlocking changes sweeping over the South during Reconstruction: to destroy the Republican party's infrastructure, undermine the Reconstruction state, reestablish control of the black labor force, and restore racial subordination in every aspect of Southern life.
Wow, look at that. The Democratic party, the same Democratic Party that has now nominated a black man for president, was once wildly backed by the militant white supremacist group, the KKK. It should also be noted that other early values of the Democratic Party were preservation of the Second Amendment Right To Bear Arms and opposition to a national bank and wealthy moneyed interests.
Here we are now, 150 years later, and it seems that the basic principles of both parties have completely reversed. The Democrats are the old Republicans, and the Republicans are the old Democrats.
It's funny how history works, isn't it?
Abraham Lincoln (Wikipedia)
Republican Party History (Wikipedia)
Democratic Party History (Wikipedia)
Klu Klux Klan (Wikipedia)
John Wilkes Booth