The Democratic and Republican parties in the United States underwent a gradual ideological realignment rather than a sudden "switch." Originally, in the mid-19th century, the Republican Party was considered more liberal, supporting abolition and a strong federal government, while the Democratic Party was conservative, agrarian, and favored states' rights, especially in the South. This was essentially the opposite of their current positions. Key points in the transition include:

• The late 19th and early 20th centuries saw some blurring of party lines, especially with Democrats like William Jennings Bryan advocating for more government intervention.
• The Great Depression in the 1930s and Franklin D. Roosevelt's New Deal significantly reshaped the Democratic Party, attracting urban workers, minorities, and liberals to form a broad coalition.
• Mid-20th century civil rights legislation (1960s), including the Civil Rights Act and Voting Rights Act, prompted many conservative Southern Democrats to shift to the Republican Party.
• By the 1980s and early 1990s, the parties had largely settled into their current ideological positions, with Republicans embracing conservatism and limited government, and Democrats leaning liberal and progressive.

Thus, the "switch" was a slow realignment of voter bases and ideologies over several decades, rather than a sudden flip—stretching roughly from the 1930s to the 1990s.