When did the ideologies of the Democratic and Republican parties flip?
There are a number of important historical reasons for this, which someone else may perhaps compile, but I will argue a simpler explanation.The Democrats have traditionally been identified as the populist party and the Republicans as the elitist party. I use both terms in value-neutral fashions and as ideological descriptors.In the 19th Century, elites believed in government intervention to build the economy while populists believed in a small government that maximized personal liberty. Slavery is a good example. Republicans saw slavery as an impediment to the economic and social development of the nation, while Democrats saw protecting slavery as a matter of personal liberty, or at least a matter of popular sovereignty.Entering the 20th Century, the spread of socialism and reform liberalism popularized the idea of government intervention as a means of promoting liberty and the public good. Teddy Roosevelt described this new idea as “the use of Hamiltonian [elitist] means to achieve Jeffersonian [populist] ends.” Therefore, populists and Democrats began to see government intervention as in their interest, and elitists and Republicans began to see government intervention as hampering economic development and inflaming class tensions.Of course, there are (many, many) exceptions to this rule, but this pattern holds for much of American history. In the past century, both liberal and conservative Republicans have often based their ideas on elitist values/ideas, while both liberal and conservative Democrats have based their ideas on populist values/ideas.I would argue that the most significant shift in American politics has occurred in the 21st Century as the parties have shifted their core organizing values from being populist and elitist to being liberal and conservative.I hope that helps!