I don't recall any official document sanctioning governments to enact social programs on behalf of the Catholic Church. Perhaps I missed it. Would appreciate a link. Thanks!
Economic Justice for All
Pastoral Letter on Catholic Social Teaching and the U.S. Economy
U.S. Catholic Bishops, 1986
http://www.osjspm.org/economic_justice_for_all.aspx
123. More specifically, it is the responsibility of all citizens, acting through their government, to assist and empower the poor, the disadvantaged, the handicapped, and the unemployed. Government should assume a positive role in generating employment and establishing fair labor practices, in guaranteeing the provision and maintenance of the economy’s infrastructure, such as roads, bridges, harbors, public means of communication, and transport. It should regulate trade and commerce in the interest of fairness [74]. Government may levy the taxes necessary to meet these responsibilities, and citizens have a moral obligation to pay those taxes. The way society responds to the needs of the poor through its public policies is the litmus test of its justice or injustice. The political debate about these policies is the indispensable forum for dealing with the conflicts and tradeoffs that will always be present in the pursuit of a more just economy.