Complex sentence: Marvin was unopposed for a second six-year term as DA in 2008, by which time he had changed his registration to Republican despite his long-time family support for the Democratic Party.
Split into simpler sentences: Marvin was unopposed for a second six-year term as DA in 2008. At some point since 2008, he switched his party affiliation to Republican despite his long-time family support for the Democratic Party.