The Republican and Democratic Party's have switched sides on the political spectrum in the past 100 years.
Early US political history
Democrat's sprung from the Anti-federalist party that was mostly agrarian, limited federal power, etc. It had traction in both the North and especially in the plantation South. They where opposed to the Federalists, who where concerned with commerce, banking, and a strong navy/federal government to protect the US merchant fleet. Federalists gained most of their support from New England, but were also represented in Virginia.
Early 19th century
Things got messy in the 1814-1860 period, but roughly speaking the Anti-Federalists/Democrats split into Jacksonian (Southern) Democrats and Northern Democrats. While the the Federalists disapeared completely and with some former members creating the shortlived National Republicans and Whig parties.
1860 Election
The Republican party was created to represent the commercial, industrial and anti-slavery agenda in the North, and had zero support in the South. The Democrats were completely split North/South letting the Republican Presidential candidate (Lincoln) win.
Post Civil War
Republicans dominated after the Civil War, and became deeply attached to the growing industrial and banking interests in the North. The rapid expansion of industry in the 1890-1929 peroid favored Republicans. Democrat's only sucsess was Woodrow Wilsons populist isolantionist stance during WWI (ironically). However, the collapse in early thirties, and the peoples disenfranchisment with industry allowed the populist Roosevelt to win the presidency.
Post WWII
The Republicans tried a different tact post-Roosevelt, positioning themselves as anti-communist hawks as opposed to Trumans percieved weakness on military matters. Eisenhower provided the perfect candidate and won (it also helped that support for US industry/economy had returned). Also, a growing North-South rift had reapeared in the Democratic party, with the Segregationist-Democrats starting to emerge.
1960 Election
This is where the old Republican=Urban/Industry/Rich, Democratic=Rural/Poor/South/Populist started to get all flipped around. Kennedy was a New Englander, and carried that region, while also taking much of the south, though his sympathy for blacks lost him the "Deep South" of Lousiana and Mississipi. Nixon was a westerner, and started to make in-roads in rural states and Appliacia. It was an odd mish-mash of an election with the old political lines gettting blurred
LBJ: The turning point.
LBJ conciously decided to finally abandon the Southern half of his party by strongly supported the Civil Rights movement, which incredibly angered the south. His remarks after signing the first Civil Rights bill was "We have lost the South for a Generation", and he was right. The Republicans took the opportunity to pounce on this turn of events and "Goldwater Republicanism" was born. Keeping the Republican staunch capitalist/Industry support, strong Christian, anti-black rights, anti-gay rights were added to platform to connect with the disenfranchised southern ex-democrats.
In 1964, the extremely popular LBJ (61% of the vote) won everywhere except the South (states which JFK had carried), where the modern idea of Republicanism had taken root.
Today
Post LBJ, the US political landscape is somewhat strange in that the Republicans now contain both a Christian/Southern/Rural wing and Rich/Libertarian/Industrial wing. The Democratics now find their support from large urban centers, labor unions, and minority's with most of their historic rural support gone. The Republican Party is currently having the same problem the Democrats had in the Truman-Kennedy era, i.e. the South. There is a growing rift between Huckabee-Republicans (Christian Southern Rural) and Gulianni-Republicans (Industrial Northern), which I think will spawn the next chapter in US political party history.