Each senator shall be elected by the legally qualified voters of the Senate district, except that if the Senate district is composed of two or more counties and two senators are apportioned to the district, one senator shall be elected by the legally qualified voters of each Assembly district. Each senator shall be elected for a term beginning at noon of the second Tuesday in January next following his election and ending at noon of the second Tuesday in January four years thereafter, except that each senator, to be elected for a term beginning in January of the second year following the year in which a decennial census of the United States is taken, shall be elected for a term of two years.