Modern Democratic and Republican parties

What is the origin and development of the modern Democratic and Republican parties? What are multiple examples that show how the constituency base has shifted over time to the present day?

This comparison examines the differences between the policies and political positions of the Democratic and Republican parties on major issues such as taxes, the role of government, entitlements (Social Security, Medicare), gun control, immigration, healthcare, abortion, environmental policy and regulation. These two parties dominate America’s political landscape but differ greatly in their philosophies and ideals.  Joe Biden vs. Donald Trump