Since the late 1850s, the United States has had two major political parties, the Democratic and Republican parties. For most of the first half of this time - from the mid-19th century to the first couple decades of the 20th - the Democrats were generally a right-wing party, espousing racism and small government, while the Republicans had a more leftist platform which strongly supported minorities1 and called for greater regulation.
From the late 1920s through the late 1970s, the two major parties essentially swapped platforms wholesale. At the beginning of the 1930s, in response to the Great Depression, the governing Republican Party disintegrated and the Democratic Party extended itself well to the left under FDR et al, with the Republicans eventually recoalescing in the mid-to-late 1940s as a center-right party opposed to labor unions and communism. Then, in the decade from the late 1960s through the late 1970s (although it was the culmination of a process that had gotten going as far back as the late 1940s), the Republicans swung hard right in order to pick up the racist-Southern-white vote and finally gain enough strength to take down the Democrats' New Deal coalition, and have continued to move further right ever since.
Nowadays, the platforms of the two main parties are essentially the reverse of what they were a century and a half ago, with the Democrats a center-to-center-left party strongly supporting big government, more regulation, and minority and labor rights, while the Republican Party strongly opposes regulation, unions, and minority rights.
Was the essentially-complete platform swap that occurred between the two major U.S. political parties in the mid-20th century a fluke, or is it common for a country's main political parties to eventually swap platforms if they survive long enough?
1: Aside from Native Americans, of course.