So "they switched sides but we refuse to call it that." Thanks.
Of course not all things considered "liberal" in the mid-19th century fit that bill today; facts on the ground have shifted enough that old labels don't necessarily apply. But in general, it's true that Republicans and Democrats swapped ideologies. By the late 19th century not much substance divided them in modern terms, especially on social issues. The changeover was complete by the late 1960s, but it really started in the late 1920s. Even today there are a few holdout "liberal Republicans" in the Northeast and "conservative Democrats" in the South, though they have lost the power they once held.