From YOUCAT, page 240:
440: Are Christians obliged to become involved in politics and society?
It is the special duty of the Christian laity people to become involved in politics, society, and commerce in spirit of the Gospel: in charity, truth and justice. Catholic social teaching offers them clear guidance in this endeavor. 
Partisan political activity is, however, incompatible with the ministry of bishops, priests, and religious, who must be of service to everyone.