Over at Against the Grain, Christopher has a long post about the concept and term "preferential option for the poor"

Catholics who are the least bit aquainted with the social doctrine of the Church have encountered the term "preferential option for the poor." According to Charles Curran, the phrase has its origins in the "liberation theology" espoused by radical Catholic theologians in Latin America (excerpt from Catholic Social Teaching Georgetown UP, 2002).

In an article for the U.S. Catholic (Why the preferential option for the poor is not optional, November 1997), Jack Jezreel chronicles the use of the phrase from a 1979 pastoral document by the Latin American Bishops, to the 1986 statement "Economic Justice for All", revisited in 1994’s "Communities of Salt and Light", as well as pontificate of Pope John Paul II.

Pope John Paul II spoke of this preferential option on many occasions, preferring the term "preferential love for the poor" —

And they are discussing it at Mirror of Justice here and here