Complex sentence: Each supervisor is required to live in his or her district, and although elections are held on a non-partisan basis, as of 2016 all 11 supervisors are members of the Democratic Party.
Split into simpler sentences: Each supervisor is elected on a non-partisan basis and is required to live in his or her district. Although supervisors' positions are non-partisan, as of 2016 all 11 supervisors are members of the Democratic Party.