Answer:
After the Second World War, after the impact that the totalitarian Nazi, Fascist and Communist regimes had caused on the American population, both Democrats and Republicans came to the conclusion that they should bet on the total fulfillment of civil and political rights in America (especially regarding racial minorities, such as African Americans).
Thus, Democrats and Republicans reached an agreement regarding this issue, by which it was agreed that both political parties would support policies, measures and movements that would tend to maximize respect for individual freedoms and equality among American citizens. This agreement was called "the Liberal consensus".