"I hope the two wings of the Democratic Party may flap together." - William Jennings Bryan
As the oldest political party in the United States, the Democratic Party has been one of the nation's major political parties for over 150 years, and diverse men and ideas have fallen under its tent since the 19th century. Today the Democrats are generally viewed as proponents of a strong, centralized federal government, and yet the forerunner of the modern party was none other than Thomas Jefferson, the man most associated with states' rights and limited government.
With its Jeffersonian background, the party championed farmers, and Andrew Jackson's populist era made the Party home to urban workers and new immigrants. Eventually sectional splits weakened the Democrats, and when the fledgling Republican Party took power under Abraham Lincoln in 1861, it ushered in an era in which the Democrats only elected 2 presidents over a 70 year span. However, Reconstruction ensured that the Democrats maintained an almost unbreakable level of support in the old Confederate states, and they used the Solid South to wield power in Congress for decades.
One hundred fifty years after the Civil War, the Democratic Party's current voting bloc (strongly reliant on minorities) and their base of power (the Northeast and Midwest) are completely different than the 19th century's incarnation. Its platform has also been completely revamped. Both of those reversals are byproducts of Franklin Roosevelt's New Deal and Lyndon B. Johnson's Great Society, which continue to be the precedents current Democratic policies spring from.