Complex sentence: While Democrats have a large advantage in registration, they are very conservative on social issues, a major reason Lewis had been able to hold on to the district with little trouble since winning it in a 1994 special election.
Split into simpler sentences: While Democrats have a large advantage in registration, the voters are very conservative on social issues. This was a major reason Lewis had been able to hold on to the district with little trouble since winning it in a 1994 special election.