The New Jersey Constitution of 1947 (Annotated) Art. IV, § II, ¶ 1

1. The Senate shall be composed of forty senators apportioned among Senate districts as nearly as may be according to the number of their inhabitants as reported in the last preceding decennial census of the United States and according to the method of equal proportions. Each Senate district shall be composed, wherever practicable, of one single county, and, if not so practicable, of two or more contiguous whole counties.