Complex sentence: At the 1980 election, the Democrats gained three more senators, giving them a total of five--a potential balance of power, which they retained until 1 July 2005, after a total lack of success at the 2004 election.
Split into simpler sentences: At the 1980 election, the Democrats gained three more senators, giving them a total of five. They held a potential balance of power, which they retained until 1 July 2005, after a total lack of success at the 2004 election.